maxInterStageShaderVariables test with unused variables

[kainino0x]

I don't think we currently have test coverage of a case where both vertex and fragment are separately under [maxInterStageShaderVariables], but would be over limit if all vertex outputs were counted as fragment inputs even if the fragment shader doesn't declare them. I think this is valid because the pipeline should elide the unused outputs?

Originally posted by @kainino0x in #4554 (comment)