LU-17761 tests: make sanity-compr sanity/sanityn return 0

LU-17761 tests: make sanity-compr sanity/sanityn return 0

While running sanity-compr sanity/sanityn, if there was
sub-subtest failure, the sanity/sanityn test_cleanup would
be incorrectly marked as FAIL.

We should leave it to the individual sanity/sanityn subtests
to mark their failures, test_sanity() and test_sanityn()
should not also return an error.

Change-Id: I1fd645b80b92e583f1a564f85e6d2d6d871b8fa8
Test-Parameters: trivial testlist=sanity-compr
Signed-off-by: Jian Yu <yujian@whamcloud.com>