tests: Divert service stdout to stderr

These tests produce structured TAP output on stdout, so we cannot run
any subprocesses that would produce unstructured stdout. Unfortunately,
by default g_debug() writes to stdout, so running with G_MESSAGES_DEBUG
to get better logging from the various subprocesses can break the test.
Use stderr instead.

Signed-off-by: Simon McVittie <smcv@debian.org>