Improve VAO extension test

This commit makes the following improvements to the VAO extension test:
-Adds testing switching between VAOs that have different number of arrays
enabled.
-Uses a color attribute instead of using a texture coordinate attribute
which is meaningless since a texture is not used.
-Refactors testing pixels to remove unnecessary indirection.
-Fixes typos and makes the test output more descriptive.
Restore VAO test pixel tolerance to 2

Tolerance was raised to 5 since the test is not testing just white and
black any more, but it's better to keep it as is unless an actual
tolerance problem can be demonstrated.
Merge pull request #628 from NVIDIA/vao-test-tweaks

Improve VAO extension test