Fixes a potential hang in test runner timeout logic.

Part of https://llvm.org/bugs/show_bug.cgi?id=25002

In writing the new process_control test included here,
I discovered that the scenario would hang indefinitely,
bypassing the timeout logic.

This fixes the indefinite hang, converting an error
in the new test to a failure.  I'll fix the failure
next.  This one was heinous enough to fix on its own,
though.

llvm-svn: 248936