Introduce testsuite

[foretspaisibles]

This introduces a testsuite, the README file shows how to run a few tests from the command line or from the REPL. There is just a few tests, to give a flavour of how they look. I believe they could be very handy to develop some advanced shape generators, such as extrusion or patching things together.

I believe it could be interesting to have some interactive testing, maybe converting the demo file into interactive testcases?

If it is possible to write the OpenGL frame in a file, it would be interesting to compare generated pictures from expected (previously generated) pictures. Not that the pictures are set in stone but reviewing changes to determine if they are legit is an interesting feature.