Synchronize access to random number generator `test_run_rand` with
a lock to ensure that `g_test_rand_*` family of functions is
thread-safe.

The reseeding taking place between test case runs is intentionally left
unsynchronized. It is an error to continue using random number generator
after test case has already finished running. Lack of synchronization
here will make such erroneous use readily apparent with thread
sanitizer.