Improve context version handling in test harness

Harness now shows clearly if the selected version of WebGL is not
supported, and test links point to the test with the right WebGL version
selected, so results are consistent whether you run the test with the
button or follow the link.
Merge pull request #667 from NVIDIA/improved-context-version-handling

Improve context version handling in test harness
Added more logging to loadImagesAsync to diagnose intermittent test failures on Chromium project's GPU bots.
Merge pull request #668 from kenrussell/more-logging

Added more logging to loadImagesAsync to diagnose intermittent test fail...