Make test failures fatal.

This was made non-fatal in the past because we want to be able to have
`libc++-test: libc++-test-target libc++-test-host`, and still run both
if the first fails, but in reality it's just hiding test failures from
the buildbot.

Change-Id: I762e7da7393a72b82f72751404dfab50919e0a74
1 file changed