Write conformance test for sampler array indexing in ESSL 3.00 shaders

ESSL 3.00 restricts the form of sampler array indexing to only allow constant expressions, unlike ESSL 1.00 which allows constant-index-expressions. A conformance test is needed which attempts to compile an ESSL 3.00 shader attempting to index a sampler array with a constant-index-expression, and assert that compilation fails.

The issue names no file or existing test. Start by locating the WebGL conformance-test entry point for ESSL 3.00 shader compilation, then add a shader using a constant-index-expression for sampler-array indexing. Done means the test compiles the shader and asserts that compilation fails.

Written by the indexing model from the issue text.