arb_tessellation_shader: add some new gl_ClipVertex tests for compat
We had GS compat tests but no tests for the tess stages.
I tested these on the Nvidia binary driver but support is totally busted there, tess shaders fail to link when they contain gl_ClipVertex with an error "unknown semantics "CLPV" specified for "gl_clipVertex" "
Edited by Timothy Arceri