rtc-test: Fix test failures with recent glib

As of glib 2.35.4, glib changed its logic for ordering test cases:

https://bugzilla.gnome.org/show_bug.cgi?id=694487

This was causing failures in rtc-test. Group the reordered test
cases into their own suite, which maintains the original ordering.

CC: qemu-stable@nongnu.org
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
