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

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

Checks that the variables generated meet certain criteria.

 - Vertex shader inputs have an explicit location.

 - Vertex shader outputs have an explicit location.

 - Fragment shader-only varying locations are not used.

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

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

 - No other kinds of vertex variables exist.

It does not verify that an specific variables exist.

v2: Fix memory management mistakes in
common_builtin::string_starts_with_prefix.  Clean up error message
reporting in common_builtin::no_invalid_variable_modes.  Both suggested
by Paul.

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