glib/tests/collate.c: run to completion when skipping all tests

Otherwise, we don't produce valid TAP output, and fail with:

    ERROR: collate - missing test plan

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=724124
Reviewed-by: Dan Winship <danw>
g_test_run: return 0 if all tests are skipped in TAP mode

Exit status 77 is special to Automake's default test driver, but is
treated as an error by TAP.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=724124
Reviewed-by: Dan Winship <danw>