button-test: Use wl_display_roundtrip instead of yield()

button-test: Use wl_display_roundtrip instead of yield()

A round trip is sufficient here.  We need to make sure that the server
has received the wl_test request and that we've received the event
that the request triggers.  The wl_display_roundtrip() helper does
exactly that: it sends a wl_display.sync request, which will hit the
server after the wl_test requests and thus the wl_callback.done event
will come back after the server has seen all the previous requests and
after we've handled all preceeding event.