Add tests for "Singular from GAP"

[heiderich]

The notebooks https://nbviewer.jupyter.org/github/oscar-system/OSCARBinder/blob/master/GAPJulia_Singular_car.ipynb and https://nbviewer.jupyter.org/github/oscar-system/OSCARBinder/blob/master/GAPJulia_Singular_blog.ipynb provide examples on how to use Singular from GAP. I propose to add test files using such kind of examples. Unfortunately those notebooks do not work anymore (see oscar-system/oscar-website#63), which illustrated the usefulness of such kind of tests.