This PR tightens our checking of stderr and will cause tests to fail if
there is any unexpected message in stderr:

  - commit Write stdout/stderr to datadir instead of temp file writes
stderr to a file in the datadir instead of a temporary file. This helps
with debugging in the case of failure.
  - commit Use LIBC_FATAL_STDERR=1 in tests ensures that libc failures
are logged to stderr instead of the terminal.

./test/functional/test_runner.py --extended