Various tests/gl_basic_test fixes

Emil Velikov requested to merge xexaxo/waffle:test-fixes into master

This series was originally inspired by the patch in !47 (closed) The MR is imported, thus it not have proper branch for the CI to kick-in.

Upon closer look it looks like the "custom" CGL tests are wrong[1]. After fixing and sorting, some missing tests became obvious. One of which triggered a seeming bug in Mesa, so I went ahead and enhanced the test. Which further highlights about a big in Mesa...

Note: the latter is an educated guess, I haven't checked against the Nvidia binary driver (yet).

[1] Based on code analysts, I don't have the HW/SW to test.

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