Make assertion in assert_ostf_run_certain more verbose

Right now if assert_ostf_run_certain fails, the output is
"AssertionError: Condition was False."
which does not help much. The change makes it more verbose
giving the details of tests which must have passed but failed.

Change-Id: Iab373c3d4809e6f27147f9331f80ba03f02ae801
(cherry picked from commit beac5f49350d99167a8c797e68d78bb63501a968)