tsan: fix trace tests on darwin

The trace tests crashed on darwin because of some thread
initialization issues (thread initialization is somewhat
different on darwin).
Instead of starting real threads, create a new ThreadState
in the main thread. This makes the tests more unit-testy
and hopefully won't crash on darwin (there is almost no
platform-specific code involved now).
This will also help with future trace tests that will need
more than 1 thread. Creating more than 1 real thread and
dispatching test actions across multiple threads in the
required deterministic order is painful.

Depends on D110539.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D110546