[Piglit] Testing GL "minimum maximums"

[Eric Anholt]

This test series is somewhat like the GLSL built-in uniforms tests I
added recently, but covering GL_MAX_* constants (and other things in
the "implementation dependent values" section or deserving to be
there) accessible through the API.  There may also be some overlap
with per-extension tests, and I think that's OK -- this is just "look
at the table in the GL spec, implement that", and means we don't have
to cross-reference with individual specs.

There are a few FINISHMEs in here.  I'm still happy enough with the
tests that I'd be ready to push them.  They catch several little
things missing for GL 3.0, and test values we've never systematically
checked for in general.

The thing I'm not really happy with is the code duplication.  I think
there's room for some helpers to get moved into util, and I think I'll
revisit that when we get to GL 3.1 testing.