arb_framebuffer_object: add missing MSAA alpha-to-coverage and alpha-to-one tests

The missing test is the one that enables both alpha-to-coverage and
alpha-to-one. It currently fails on AMD drivers, but passes on llvmpipe.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <!978>