Corrections to the sRGB test based on careful re-reading of the spec

It seems that the initial revisions of this test were built on an inaccurate reading of the sRGB extension spec. SRGB_EXT should not be a renderable texture format, but SRGB_ALPHA_EXT should. Updated the test to reflect this and ensure that the right errors are thrown when attempted to use SRGB_EXT render targets.