Fix flaky test: signal_in_mutex_lock.cpp

Fix flaky test: the spawned thread keeps spinning
on `sampler_mutex` which may be released before
the thread is terminated based on termination
ordering.

My understanding of C++ semantics are that the
program here is invalid: the destructors of global
variables are invoked at the time of program
termination, and it is the responsibility of the
program to ensure that invoking those destructors
is safe.

rdar://126768628