tests,libtracker-common: fix tests expecting stdout/stderr messages

We need to make sure that tests expecting messages from stdout/stderr are not
dependent on the specific value set for G_MESSAGES_DEBUG. In other words, make
sure that the trap fork tests dump to stdout/stderr unconditionally.

Before this fix, we had:

$ ./tracker-dbus
    /libtracker-common/tracker-dbus/slist_to_strv_ok: OK
    /libtracker-common/tracker-dbus/request: **
    Tracker:ERROR:tracker-dbus-test.c:131:test_dbus_request: stdout of child process (12948) failed to match: *TestNewOK*
    Aborted (core dumped)

$ G_MESSAGES_DEBUG=all ./tracker-dbus
    /libtracker-common/tracker-dbus/slist_to_strv_ok: OK
    /libtracker-common/tracker-dbus/request: Tracker-DEBUG: <--- [1|0] Test request (--TestNewOK--))
    OK
    /libtracker-common/tracker-dbus/request_failed: Tracker-DEBUG: <--- [2|0] test_dbus_request_failed()
    OK

Now, we just have:

$ ./tracker-dbus
    /libtracker-common/tracker-dbus/slist_to_strv_ok: OK
    /libtracker-common/tracker-dbus/request: OK
    /libtracker-common/tracker-dbus/request_failed: OK

https://bugzilla.gnome.org/show_bug.cgi?id=710092