Attempt to fix build failure on OS X and Debian alpha; the symptom is

Attempt to fix build failure on OS X and Debian alpha; the symptom is
consistent with os.wait() returning immediately because some other
subprocess had previously exited; the test suite then immediately
tries to lock the mailbox and gets an error saying it's already
locked.

To fix this, do a waitpid() so the test suite only continues once
the intended child process has exited.