test_osutils.TestWalkDirs.test_walkdirs_encoding_error fails on Mac OSX

[Martin Packman]

As seen on babune Mac bulders, the test added for bug 488519 fails:

<http://babune.ladeuil.net:24842/job/selftest-osx-10.5/41/testReport/bzrlib.tests.test_osutils/TestWalkDirs/test_walkdirs_encoding_error/>

Traceback (most recent call last):
  ...
  File ".../bzrlib/tests/test_osutils.py", line 1120, in test_walkdirs_encoding_error
    self.assertRaises(errors.BadFilenameEncoding, attempt)
AssertionError: BadFilenameEncoding not raised

Is it the case that HFS will silently mangle non-utf-8 bytes into utf-8 rather than create an invalidly named file? In which case, there needs to be a more specific feature than ByteStringNamedFilesystem for old-style anything goes unix filesystems used for this test instead.