[tsan] Call pthread interceptors directly in TSan RTL unit tests

On Darwin, we're running the TSan unit tests without interceptors. To make sure TSan observes all the pthread events (thread creating, thread join, condvar signal, etc.) in tsan_posix.cc, we should call the pthread interceptors directly, as we already do in tsan_test_util_posix.cc. This fixes some flaky failures on Darwin bots.

Differential Revision: https://reviews.llvm.org/D26639

llvm-svn: 287026