Attempt to fix flaky tests.

The threaded cxa guard test attempted to test multithreaded waiting
by lining up a bunch of threads at a held init lock and releasing them.
The test initially wanted each thread to observe the lock being held,
but some threads may arive too late.

This patch cleans up the test and relaxes the restrictions.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@359785 91177308-0d34-0410-b5e6-96231b3b80d8