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

[kenrussell]

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.