gio/tests/file: skip the file monitor tests if using GPollFileMonitor

gio/tests/file: skip the file monitor tests if using GPollFileMonitor

test_create_delete() assumes that if it creates a file and then
immediately deletes it, that the file monitor will notice this and
record it as a create followed by a delete. But that won't work with
GPollFileMonitor, which will just think nothing changed. So skip the
test in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=669331