tests/avocado/machine_s390_ccw_virtio: Skip the flaky virtio-gpu test by default

tests/avocado/machine_s390_ccw_virtio: Skip the flaky virtio-gpu test by default

The virtio-gpu test is known to be flaky - that's why we also did
not enable the test_s390x_fedora in the gitlab CI. However, a flaky
test can also be annoying when testing locally, so let's rather skip
this subtest by default and start running the test_s390x_fedora test
in the gitlab CI again (since the other things that are tested here
are quite valuable).

Message-Id: <20230724084851.24251-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>