Check results of one test in run_single_ostf_test

In 'run_single_ostf_test' method we run only one health
check for environment and expect that it will pass. So
we should verify only its results, because some other OSTF
tests сould fail earlier.

Change-Id: Ibb0d38a10fda4f1250b78e7fb61dd66bdc4505d6
Closes-bug: #1354300
Merge "Check results of one test in run_single_ostf_test"