A word to the wise:

You must ensure that if your test finishes successfully, no processes have
priority FOREGROUND_HIGH.

If you don't do this, expect to see tests randomly fail with mysterious
FOREGROUND --> FOREGROUND priority transitions.

What's happening in this case is that your FOREGROUND_HIGH process lives until
the beginning of the next test.  This causes the process started by the next
test to have low CPU priority.  Then the FOREGROUND_HIGH process dies, because
its iframe gets GC'ed, and we transition the new test's process from low CPU
priority to regular CPU priority.

Ouch.