Upstream unused uniform fixes to ES 3.0 suite, make harness stricter

[kenrussell]

In #340 the WebGL conformance suite harness for running the imported ES 3.0 tests was relaxed to be identically as strict as ES 3.0's GTF harness, because of problems with unreferenced uniforms being optimized away by some GLSL compilers. This was an expedient workaround for allowing the WebGL 1.0.2 conformance suite to be passed.

A better fix would be to fix the tests and make the ES 3.0 conformance suite harness stricter, requiring that all referenced uniforms be present. At that point the WebGL harness should me made stricter as well.

This is a follow-on to Issue 236.