LU-13081 tests: skip sanity test_151/test_156

LU-13081 tests: skip sanity test_151/test_156

Skip both sanity test_151 and test_156 during interop testing,
since this is really testing server-side functionality only
(OSS caching behavior). And it makes sense to just exclude
test_151 and test_156 during interop testing, otherwise it
seems that the client version of the test can become
inconsistent with the caching behavior/tunables on the OSS
and the failures don't mean anything. There is enough
non-interop testing to catch any regressions in the OSS
cache behavior.

Lustre-change: https://review.whamcloud.com/50777
Lustre-commit: TBD (from 62cd8d19ff103e6e8a2b4bb7cdc00815ddb0edac)

Test-Parameters: trivial
Signed-off-by: Alex Deiter <adeiter@tintri.com>
Change-Id: I39a8b54894d5b0c7573e6c56d1f8e1ba02b3e3fe
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/50887
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>