Add a GLSL test that compares a loop index to an uniform

Comparing a loop index to an uniform is useful in WebGL GLSL shaders,
since it can be used to work around the lack of while loops. Add a test
for this. The test is known to have failed on at least some older NVIDIA
Tegra drivers.
Merge pull request #388 from NVIDIA/compare-loop-index-test

Add a GLSL test that compares a loop index to an uniform