arb_tessellation_shader: add some new gl_ClipVertex tests for compat

Timothy Arceri requested to merge tarceri/piglit:tess_clip_vertex into master

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

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