tsan: deflake a test

There was a failure on a bot:
http://lab.llvm.org:8011/builders/clang-cmake-mipsel/builds/1283

strerror test is indeed flaky. We protect all races by a barrier in other tests
to eliminate flakiness. Do the same here.

No idea why tls_race2.cc failed. Add output at the end of the test
as we do in other tests. Sometimes test process crashes somewhere
in the middle (e.g. during race reporting) and it looks like empty output.
Output at the end of test allows to understand if the process has crashed,
or it has finished but produced no race reports.

Reviewed in https://reviews.llvm.org/D42633

llvm-svn: 323657