tests: Fix a double-close of a GSource in tests

This bug only gets triggered on BSD, because it calls recv_message() with G_IO_HUP | G_IO_IN. That takes two code paths in recv_message(), and ends up calling shutdown_source() twice. The second call causes a critical warning.

Fixes: https://gitlab.gnome.org/GNOME/glib/-/jobs/2004192

Signed-off-by: Philip Withnall pwithnall@endlessos.org

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