Create test case for UBO update bug in ANGLE's Metal backend

In crbug.com/338348430 a bug was reported against ANGLE's Metal backend where updates to a uniform buffer object in between draw calls wasn't having the intended effect. A C++ test case was written for it; that test needs to be ported to the WebGL conformance suite in order to ensure this doesn't regress again in any browser or on any platform.

Locate the existing C++ test for crbug.com/338348430 and the WebGL conformance suite, then compare how the bug scenario is represented in each. Port the test case so the uniform buffer update between draw calls is covered, and verify that the conformance test detects the regression across browsers or platforms.

Written by the indexing model from the issue text.