Update platform_CrosDisksFilesystem ext2/3/4 tests.

CL:209165 drops the support of ext2/3/4 filesystem in cros-disks. This
CL updates platform_CrosDisksFilesystem ext2/3/4 tests to verify that
mounting of ext2/3/4 filesystems via cros-disks results in an
'unsupported filesystem error'.

BUG=chromium:315401
CQ-DEPEND=CL:209165
TEST=Run platform_CrosDisksFilesystem tests.

Change-Id: Ie5d341632b9da6628d782fc86e76514d0eb37589
Reviewed-on: https://chromium-review.googlesource.com/209152
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
4 files changed