Fix set up of queue-signal.exp test.

The test does a backtrace to see which thread (#2 or #3) is assigned
to which SIGUSR (1 or 2).  If the main thread gets to all_threads_running
before the sigusr threads get to their entry point, then the function
name isn't in the backtrace and the test fails.

Alas this version of the code is within epsilon of what I started with,
and then over-simplified things.