!UPSTREAM: gs: Test all core inputs except gl_PrimitiveIDIn.

gl_PrimitiveIDIn will need more aggressive testing, so that will be
addressed in a separate patch.

TODO:
- Add core outputs (gl_ClipDistance already done)
gs: Test gl_PrimitiveIDIn
glsl-1.20: Test that a literal array of structs can be dereferenced.

A bug currently in Mesa (as of commit b616d01) causes these tests to
segfault.