glsl/tests: Verify fragment shader built-ins generated by _mesa_glsl_initialize_variables

glsl/tests: Verify fragment shader built-ins generated by _mesa_glsl_initialize_variables

Checks that the variables generated meet certain criteria.

 - Fragment shader inputs have an explicit location.

 - Fragment shader outputs have an explicit location.

 - Vertex / geometry shader-only varying locations are not used.

 - Fragment shader uniforms and system values don't have an explicit
   location.

 - Fragment shader constants don't have an explicit location and are
   read-only.

 - No other kinds of fragment variables exist.

It does not verify that an specific variables exist.

v2: Use _mesa_varying_slot_in_fs in
fragment_builtin.inputs_have_explicit_location.  Suggested by Paul.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>