Previously this module only provided `GLUT_INCLUDE_DIR`, which does not
follow the modern naming convention documented in `cmake-developer(7)`.
Issue: #23018
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