gdx-tests: GlTexImage2D and KTXTest don't work with gl30

GlTexImage2D (the test, not the method) just has a black screen. If logGLErrors is true, it spews [GLProfiler] error 1282 (Invalid Operation).

KTXTest has error 1280 (Invalid Enum) and crashes:

Exception in thread "main" com.badlogic.gdx.utils.GdxRuntimeException: Fragment shader:
0(21) : error C7531: global function textureCube requires "#extension GL_NV_shadow_samplers_cube : enable" before use

I think both need shader changes.

This is probably already known and I don't think it really matters: Gdx2DTest and TextureFormatTest differ between gl30 and gl20 due to the use of Alpha and LuminanceAlpha formats.