tests: Link virtestmock with probes

tests: Link virtestmock with probes

I've encountered this error while trying out this feature on some
systems:

  $ VIR_TEST_FILE_ACCESS=1 ./virhashtest \
 libvirt.git/tests/.libs/lt-virhashtest: \
symbol lookup error: libvirt.git/tests/.libs/virtestmock.so: \
undefined symbol: libvirt_event_poll_purge_timeout_semaphore

Problem is, linking just libvirt_utils to virmock.la is not
enough. We might need to link libvirt_probes.lo too.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>