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build: Clean files left behind after gio/tests/gresource.c test

This is a follow-up to commit 614adf8a,
which started generating two new files as part of the test; they need to
be cleaned up before distcheck will pass.

Ideally, the test should run a temporary directory and wipe that
directory itself before exiting, but that’s a bit of a big change to
make right now. Deferred to
GNOME/glib#1495.

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