mesa: Cut enum_strings.cpp test down to a few hand-chosen enums.

mesa: Cut enum_strings.cpp test down to a few hand-chosen enums.

The previous contents appeared to be the output of some form of code
generation for all enums, with a few entries hand-edited to deal with
oddness.  The downside to this was that when an enum gets promoted from
vendor to _EXT or _EXT to _ARB or _ARB to core, make check starts failing
even when the commiter has done nothing wrong.  Instead of black-box
testing the code generation, pick a few enums that intentionally poke the
interesting cases of code generation.

People editing the code generator should be diffing the generated code
anyway.  This should catch when they fail to do so, without throwing false
negatives when people update the GL XML.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>