tests/gl_basic_test: complete and document test coverage

From the inline comment:

Quick notes which combinations we test and why
 - w/o version, for each API/profile combination check variations of
   - visual - make sure we can draw fine, duh
   - flags - flags selection machinery is funky
 - fwdcompat was introduced with OpenGL 3.0, check it and 3.1
 - specific version
   - all known versions
   - no permutation

As such we:
 - drop 21+fwdcompat, and
 - add compat+fwdcompat
 - add gl core/compat 4.4-4.6
 - add gles3 3.1-3.2

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
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