Include the new garbage test resource

The installed-test fail without that, see https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-cosmic/cosmic/amd64/g/glib-networking/20180817_140840_ce608@/log.gz

"/tls/certificate/create-with-garbage-input: ** GLib-Net:ERROR:../tls/tests/certificate.c:237:test_create_certificate_with_garbage_input: assertion failed (error == (g-tls-error-quark, 2)): Failed to open file ?/usr/lib/glib-networking/installed-tests/glib-networking/files/garbage.pem?: No such file or directory (g-file-error-quark, 4) FAIL: glib-networking/certificate.test (Child process killed by signal 6)"