Need test of clearColor and color channels

In https://bugs.webkit.org/show_bug.cgi?id=222844 @grorg points out that there are *no* WebGL conformance tests which verify that the color channels passed to clearColor are correctly handled. All of the tests apparently clear to grayscale values.

This is an obvious testing gap which must be fixed.

Start by locating the existing WebGL conformance tests for clearColor and checking how their current grayscale cases are structured. Add coverage for the individual color channels and verify that the conformance tests distinguish the supplied channel values correctly; done means the new tests run with the existing suite.

Written by the indexing model from the issue text.