tests: don't delay/skip frames due to glReadPixel concerns

tests: don't delay/skip frames due to glReadPixel concerns

This greatly speeds up running all the conformance tests by no longer
delaying many of the tests for a number of dummy frames to be painted.

We used to skip frames because we thought there was a problem with the
driver's glReadPixels implementation. Although we have seen driver
issues at times the real reason the delay was needed was because
resizing the stage usually happens asynchronously (because a non
synchronous X request is used by clutter_stage_set_size()). We now force
all X requests to be synchronized for the conformance tests so this is
no longer a problem and we can avoid these hacks.