Lab - Test the external plugin system

[janetournois]

Issue Details

The external plugin framework for CGAL lab should be tested in the testsuite - at least for compilation.

[afabri]

What exactly do you want to be tested?

[janetournois]

I want the system of external plugin of CGAL Lab, with which you can have your own plugin in a separate repository, and define the environment variable POLYHEDRON_DEMO_PLUGINS_PATH to be tested, to make sure that it remains functional.