Add unit test to verify that modifying uniforms doesn't create chain

The recommended usage model for rendering pipelines with minor changes
is to make a copy of a base pipeline just before rendering and then
modify that. The new pipeline can then be used as the base pipeline
for the next paint. Currently this has a known problem when modifying
uniform values in that Cogl won't prune the redundant ancestry and
instead it will end up with an ever-growing chain of pipelines. This
is particularly bad for something like CoglGST where it could also end
up leaking textures for the video frames if the pipelines are used to
render video.

The patch adds a test case for that situation so that we won't forget
about the problem. The test is maked as a known failure. Additionally
the patch adds a similar test for setting the blend constant to
constrast the test with some state that does work correctly.

Reviewed-by: Robert Bragg <robert@linux.intel.com>