tests: remove skip() and share set_opaque_rect()

Two misc patches I created when looking to expand another test.

I have a funny feeling and I left the skip() function in tree for some reason, but a quick search didn't find that reason recorded.

set_opaque_rect() is preparing to copy alpha-blending test into the color-matrix-shaper test, but a nice detail there is that struct surface gains a pointer to client so you no longer need to carry both if you do something with a surface. Maybe that's generally useful in the future.