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>