Fail selftest if anything is written to stderr

The new ResourceWarning exceptions in Python 3.8 are ignored since
they're raised during garbage collection, which makes them easy to
miss even with PYTHONWARNINGS=error in our testing. Luckily, their
tracebacks are written to stderr, and we normally expect nothing
will be written to stderr on successful runs of the selftest script.
As a simple solution to avoid similar situations in the future,
cause the selftest run to fail if anything at all gets written to
the service's stderr.