Add test to cover ANGLE regression with HLSL compiler settings

Forked from http://anglebug.com/5094

The devs shared a small reproduction (attached) that should be made into a WebGL test to ensure functionality doesn't regress again.

[sample.html.zip](https://github.com/KhronosGroup/WebGL/files/5279265/sample.html.zip)

Start by opening the attached sample.html.zip and tracing the ANGLE/HLSL compiler-settings scenario described by the reproduction. Add a WebGL test covering that regression, then verify the test demonstrates the expected behavior and prevents the issue from recurring.

Written by the indexing model from the issue text.