Add test of manual mipmap chain generation in WebGL 2.0

After the updates from https://anglebug.com/4690 , ANGLE guarantees support for allocating a texture in WebGL 2.0 via `TexStorage2D` and manually generating mipmaps by iteratively rendering to level N+1 while sampling at level N. (Specifically, that this is not considered a rendering feedback loop.)

A test that this works has been added to ANGLE's test suite, but it should be ported to JavaScript and added to the WebGL 2.0 conformance test suite to guarantee that this functionality works across browsers.

Start by locating the existing ANGLE test for manual mipmap chain generation and the WebGL 2.0 conformance test structure. Port that coverage to JavaScript, exercising TexStorage2D and iterative rendering between mipmap levels; done means the test passes in the conformance suite across browsers.

Written by the indexing model from the issue text.