adt_run test_tree_output_dir always fails on armhf

[Dan Streetman]

[impact]

this autopkgtest always fails on armhf

[test case]

run autopkgtest on armhf:

======================================================================
FAIL: test_tree_output_dir (__main__.NullRunner)
source tree, explicit --output-dir
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/autopkgtest.SJn7ZH/build.9Ef/src/tests/adt-run", line 454, in test_tree_output_dir
    self.assertIn('cpu_model', info)
AssertionError: 'cpu_model' not found in {'custom_environment': ['ADT_COLOR=blue', 'ADT_SOMETHING=foo bar'], 'kernel_version': 'Linux 4.15.0-62-generic #69-Ubuntu SMP Wed Sep 4 20:57:49 UTC 2019', 'virt_server': 'adt-virt-null', 'cpu_flags': 'fp asimd evtstrm cpuid', 'nproc': '4'}

[regression potential]

test case fix only, low

[other info]

this specific check is limited on bionic and later to amd64/i386; on xenial it's also run on armhf for some reason, but it always fails. It should not be run on armhf.