LU-10589 tests: insulate sanity-dom from test failures

LU-10589 tests: insulate sanity-dom from test failures

The test suite sanity-dom calls two other test suites.
If one of these subtest suites has a test failure, that
failure is correctly reported as a failure, but it also
causes the last test in the subtest suite to look like
a failure.

Return zero for the subtest suites sanity and sanityn.

Test-Parameters: trivial fstype=zfs testlist=sanity-dom
Test-Parameters: fstype=ldiskfs testlist=sanity-dom
Test-Parameters: mdscount=2 mdtcount=4 fstype=zfs testlist=sanity-dom
Test-Parameters: mdscount=2 mdtcount=4 fstype=ldiskfs testlist=sanity-dom
Signed-off-by: James Nunez <jnunez@whamcloud.com>
Change-Id: I778990ad8724de182a96399cf09758edfd35b2e1
Reviewed-on: https://review.whamcloud.com/36369
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>