gtestutils: Print non-matching stderr/stdout output on trap failure
gtestutils: Print non-matching stderr/stdout output on trap failure

When running a test as a subprocess and matching its output, it’s very
annoying for GLib to tell you that the output didn’t match your pattern,
*but not actually say what the output was*. Fix that.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib