Improve unit test output and logging to diagnose build breaks

Improve unit test output and logging to diagnose build breaks

Summary:
I can't repro the very frequent breakage we get from the unit
test framework, and it would be helpful to get the proper error messages
from it, so this adjusts the logging and changes a CHECK to an EXPECT to
get more data when the failure occurs.

Test Plan: runtests

Reviewed By: hans@fb.com

Subscribers: lins, anca, folly-diffs@

FB internal diff: D1761800

Tasks: 5870141

Signature: t1:1761800:1420226753:27a66fb0ce47cd57529e5fd0518550fb649c9eb5