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 could fail earlier.

Change-Id: Ibb0d38a10fda4f1250b78e7fb61dd66bdc4505d6
Closes-bug: #1354300
(cherry picked from commit c149d919e7eb3cbc228158175ec6f2cfa8ffd982)
Merge "Check results of one test in run_single_ostf_test" into stable/5.0