tests: Divert service stdout to stderr
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 <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@debian.org>

matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal