Add texStorage2D test

This adds a basic test for texStorage2D and its interaction with other texture image specification methods. Mostly error status checking, and some basic rendering checking.

Note that since texStorage2D does not accept the legacy way of specifying (unsized internalformat, type) pairs, and only accepts sized internalformats, this also exercises this aspect of a WebGL2 implementation. And while we were at it, this exercises some of the crazy new formats!

(A much more comprehensive test specifically about testing new formats and the new possibility of (unsized format) != (sized internalformat) should follow soon.
Could you think of a way to automatically generate more test cases so that this test can cover more of the new texture formats?