tsan: for unittests, change to use test fixtures to clear racy stacks

tsan: for unittests, change to use test fixtures to clear racy stacks

After cd0a5889d71, unittest would run in shard mode where many tests
share a single process. Need to clear some global state to make the test
results stable.

Reviewed By: thetruestblue, rsundahl

Differential Revision: https://reviews.llvm.org/D124591