Use smaller textures in compressed texture size limit test

The patches limit tested cube map texture size to 2048x2048 according to discussions on the mailing list. Getting some test coverage of huge cube maps back might be possible in some way, but requires policy decisions around OOM and context loss handling.

The patches also optimize testing TEXTURE_2D further by using rectangular compressed textures instead of square ones, and take ArrayBuffer maximum size into account, which might become relevant at some point in the future.