tsan: add another fork test

Add a fork test that models what happens on Mac
where fork calls malloc/free inside of our atfork
callbacks.

Reviewed By: vitalybuka, yln

Differential Revision: https://reviews.llvm.org/D114250

GitOrigin-RevId: 6a3958247aeeacdbf40833151220b089f066c82f
2 files changed