FindGLUT: Provide legacy GLUT_INCLUDE_DIR result in pkg-config code path

Since commit f90d15458a (FindGLUT: Use pkg-config to find flags if
available, 2021-06-11, v3.22.0-rc1~469^2) we return early if pkg-config
provides the information.  During review of that commit, code to
populate the legacy `GLUT_INCLUDE_DIR` result variable was removed from
that code path.  Add code to provide it.

Also fix the test case to use `GLUT_INCLUDE_DIR`, the result variable
documented officially by the module.

Fixes: #23018