glsl/tests: Add first simple tests of populate_consumer_input_sets

glsl/tests: Add first simple tests of populate_consumer_input_sets

Four initial tests:

* Create an IR list with a single input variable and verify that
  variable is the only thing in the hash tables.

* Same as the previous test, but use a built-in variable
  (gl_ClipDistance) with an explicit location set.

* Create an IR list with a single input variable from an interface block
  and verify that variable is the only thing in the hash tables.

* Create an IR list with a single input variable and a single input
  variable from an interface block.  Verify that each is the only thing
  in the proper hash tables.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>