[compiler-rt] [tsan] Test `__tsan_test_only_on_fork` only on Mac (PR #96597)

[Vitaly Buka via llvm-commits]

vitalybuka wrote:

I am going to land dependent changes on top of this patch.

If this breaks stuff on some Apple bots, instead of reverting, please just mark the test as UNSUPPORTED, and let me know.


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