[llvm-branch-commits] [tsan] Test `__tsan_test_only_on_fork` only on Mac (PR #96597)

[Vitaly Buka via llvm-branch-commits]

https://github.com/vitalybuka created https://github.com/llvm/llvm-project/pull/96597

According to https://reviews.llvm.org/D114250
this was to handle Mac specific issue, however
the test is Linux only.

The test effectively prevents to lock main allocator
on fork, but we do that on Linux for other
sanitizers for years, and need to do the same
for TSAN to avoid deadlocks.