LU-15131 tests: check parameter correctly

LU-15131 tests: check parameter correctly

sanity test_254() always skipped due to wrong
parameter path: MDT0 not initialized before get_param.

Fixes: 33aad7829de5 ("LU-10461 tests: call exit in the skip routine")
Test-Parameters: trivial testlist=sanity
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
HPE-bug-id: LUS-9969
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Change-Id: I02e38a27ca4656128e62339d63425f85386fa905
Reviewed-on: https://review.whamcloud.com/45300
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>