tests: Mark refcount/properties2 test as slow
tests: Mark refcount/properties2 test as slow

It often takes a bit more than 30s to run on my local machine.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib