gdb/testsuite: fix race condition in gdb.python/py-thread-exited.exp

gdb/testsuite: fix race condition in gdb.python/py-thread-exited.exp

I ran into a test failure on gdb.python/py-thread-exited.c.  The test
creates two threads and then catches the thread exits in Python.  The
test expects the threads to exit in a specific order.

As the test is currently written, it is _likely_, but not guaranteed,
that the threads will exit in the same order they are created, which
is what the test expects.

When running on a loaded system I ran into a case where the threads
exited in the reverse creation order, which caused the test to fail.

I could fix this by having the .exp file not care about the thread
order, or by changing the C file to force the order. I chose the
later, and added a pthread_barrier_t to ensure the threads exit in the
correct order.

There should be no change in what is tested after this commit.