Updated the EXT_sRGB conformance test to test more code paths.

New tests:

 * Ensures that sRGB textures cannot be created if the extension hasn't been enabled
 * Validates that the constant values attached to the sRGB extension object match the ones from the spec.
 * Tests attaching an SRGB_ALPHA_EXT texture to a framebuffer (previously only tested SRGB_EXT)