LU-6707 tests: Add ability to skip tests in POSIX

LU-6707 tests: Add ability to skip tests in POSIX

Although the POSIX test suite only has a single sub test,
there are some times when you may need to exclude this
test from being run.

We need to add the ability to skip subtests in the POSIX
test suite.

Test-Parameters: trivial
Signed-off-by: James Nunez <james.a.nunez@intel.com>
Change-Id: Idb051bc7ced02c9f09190874b5e7dacb3c6ad6ce
Reviewed-on: https://review.whamcloud.com/27012
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Saurabh Tandan <saurabh.tandan@intel.com>
Reviewed-by: Wei Liu <wei3.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>