With FEATURES="test -usersandbox" I get these errors:
[13:56:10] mysys/waiting_threads-t ..................................... ok        4 ms
[13:56:10] mysys/base64-t .............................................. ok      154 ms
[13:56:10] mysys/lf-t .................................................. ok      155 ms
[13:56:10] mysys/my_atomic-t ........................................... ok       62 ms
[13:56:10] mysys/bitmap-t .............................................. ok      289 ms
[13:56:10] strings/strings-t ........................................... ok        1 ms
[13:56:10] ../storage/maria/unittest/ma_pagecache_consist_64kHC-t ...... ok        8 ms
[13:56:10] ../storage/maria/unittest/ma_pagecache_consist_64k-t ........ ok        3 ms
[13:56:10] ../storage/maria/unittest/ma_test_loghandler_nologs-t ....... ok      299 ms
[13:56:11] ../storage/maria/unittest/ma_test_loghandler_noflush-t ...... ok       19 ms
[13:56:11] ../storage/maria/unittest/ma_pagecache_consist_1k-t ......... ok        8 ms
[13:56:11] ../storage/maria/unittest/ma_test_loghandler_long-t ......... ok     3861 ms
[13:56:15] ../storage/maria/unittest/ma_pagecache_consist_1kHC-t ....... ok        7 ms
[13:56:15] ../storage/maria/unittest/ma_test_loghandler_readonly-t ..... ok       55 ms
[13:56:15] ../storage/maria/unittest/ma_test_loghandler_purge-t ........ ok      256 ms
[13:56:15] ../storage/maria/unittest/ma_test_loghandler_max_lsn-t ...... ok      269 ms
[13:56:15] ../storage/maria/unittest/ma_test_loghandler_first_lsn-t .... ok       21 ms
[13:56:15] ../storage/maria/unittest/ma_pagecache_consist_64kRD-t ...... ok    33304 ms
[13:56:49] ../storage/maria/unittest/ma_pagecache_consist_1kRD-t ....... ok      443 ms
[13:56:49] ../storage/maria/unittest/ma_pagecache_single_1k-t .......... ok      333 ms
[13:56:49] ../storage/maria/unittest/ma_test_loghandler_multiflush-t ... ok     5572 ms
[13:56:55] ../storage/maria/unittest/ma_test_loghandler_pagecache-t .... ok       22 ms
[13:56:55] ../storage/maria/unittest/ma_test_loghandler_multithread-t .. ok    29585 ms
[13:57:25] ../storage/maria/unittest/ma_test_loghandler_multigroup-t ... ok    27926 ms
[13:57:53] ../storage/maria/unittest/ma_test_loghandler-t .............. ok     4067 ms
[13:57:57] ../storage/maria/unittest/ma_pagecache_rwconsist2_1k-t ...... ok        6 ms
[13:57:57] ../storage/maria/unittest/ma_pagecache_rwconsist_1k-t ....... ok        6 ms
[13:57:57] ../storage/maria/unittest/ma_pagecache_consist_64kWR-t ...... ok    43644 ms
[13:58:40] ../storage/maria/unittest/ma_pagecache_consist_1kWR-t ....... ok     1317 ms
[13:58:42] ../storage/maria/unittest/ma_pagecache_single_8k-t .......... ok     1846 ms
[13:58:44] ../storage/maria/unittest/ma_pagecache_single_64k-t ......... ok        8 ms
[13:58:44] ../storage/maria/unittest/ma_control_file-t ................. ok        1 ms
[13:58:44] ../storage/maria/unittest/trnman-t .......................... ok      687 ms
[13:58:44] ../storage/maria/unittest/ma_test_all-t ..................... 77/830 
Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e4096 -K ' (loop iteration 8.) at line 316
(errcode: 256, test: 83)
[13:58:44] ../storage/maria/unittest/ma_test_all-t ..................... 85/830 
Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e8192 -K ' (loop iteration 9.) at line 316
(errcode: 256, test: 85)

Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e16384 -E16384 -K -L ' (loop iteration 10.) at line 316
(errcode: 256, test: 87)
[13:58:44] ../storage/maria/unittest/ma_test_all-t ..................... 238/830 
Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e4096 -K -S' (loop iteration 8.) at line 316
(errcode: 256, test: 242)

Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e8192 -K -S' (loop iteration 9.) at line 316
(errcode: 256, test: 244)
[13:58:44] ../storage/maria/unittest/ma_test_all-t ..................... 246/830 
Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e16384 -E16384 -K -L -S' (loop iteration 10.) at line 316
(errcode: 256, test: 246)
[13:58:44] ../storage/maria/unittest/ma_test_all-t ..................... 399/830 
Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e4096 -K -M' (loop iteration 8.) at line 316
(errcode: 256, test: 401)

Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e8192 -K -M' (loop iteration 9.) at line 316
(errcode: 256, test: 403)

Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e16384 -E16384 -K -L -M' (loop iteration 10.) at line 316
(errcode: 256, test: 405)
[13:58:44] ../storage/maria/unittest/ma_test_all-t ..................... 556/830 
Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e4096 -K -M -T' (loop iteration 8.) at line 316
(errcode: 256, test: 560)

Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e8192 -K -M -T' (loop iteration 9.) at line 316
(errcode: 256, test: 562)
[13:58:44] ../storage/maria/unittest/ma_test_all-t ..................... 564/830 
Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e16384 -E16384 -K -L -M -T' (loop iteration 10.) at line 316
(errcode: 256, test: 564)
[13:58:44] ../storage/maria/unittest/ma_test_all-t ..................... 719/830 
Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e4096 -K -M -T -C' (loop iteration 8.) at line 316
(errcode: 256, test: 719)

Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e8192 -K -M -T -C' (loop iteration 9.) at line 316
(errcode: 256, test: 721)

Failed test '../storage/maria/unittest/../ma_test2 -s -htmp -m10000 -e16384 -E16384 -K -L -M -T -C' (loop iteration 10.) at line 316
(errcode: 256, test: 723)
[13:58:44] ../storage/maria/unittest/ma_test_all-t ..................... Dubious, test returned 1 (wstat 256, 0x100)
Failed 15/830 subtests 
[14:00:37] ../dbug/tests-t ............................................. skipped: because dbug is disabled
[14:00:37]

Test Summary Report
-------------------
../storage/maria/unittest/ma_test_all-t                   (Wstat: 256 Tests: 830 Failed: 15)
  Failed tests:  83, 85, 87, 242, 244, 246, 401, 403, 405
                560, 562, 564, 719, 721, 723
  Non-zero exit status: 1
Files=35, Tests=145284, 267 wallclock secs ( 8.93 usr  0.30 sys + 138.85 cusr 56.43 csys = 204.51 CPU)
Result: FAIL
Failed 1/35 test programs. 15/145284 subtests failed.

Could it be because I have /var/tmp/portage on a tmpfs?