Fix exit status of `run` scripts executing more than one test.

For some ART run-tests, the `run` script executes more than one
instance of the test (e.g. test/004-ThreadStress/run executes the
ThreadStress test twice: the first time as a normal run, the second
time in locks-only mode with stack-dump lock profiling). However,
these tests were returning the exit status of last test executed,
which could hide a potential earlier failure. This change ensure we
return the first failing exit status, if any.

Test: art/test/testrunner/testrunner.py
Change-Id: I5e4e4cc7d9311fe15637ea2f5248a0e9f2432d61