[Compiler-rt] Fix running ASan/TSan unit tests under macOS 12.0.

On macOS the unit tests currently rely on libmalloc being used for
allocations (due to no functioning interceptors) but also having the
ASan/TSan allocator initialized in the same process.

This leads to crashes with the macOS 12.0 libmalloc nano allocator so
disable use of the allocator while running unit tests as a workaround.

rdar://80086125

Differential Revision: https://reviews.llvm.org/D107412