Add test coverage

As https://github.com/openai/gym/pull/2789 is becoming a dead-end because of Github permissions issues, I started to look into other solutions from https://github.com/openai/gym/issues/2734, like the ones below:
1. [coverage.py badge](https://github.com/marketplace/actions/coverage-py-badge)
2. [genbadge](https://smarie.github.io/python-genbadge/)

None of them is perfect, but here are the main differences:
1. Drawback: the Git history will be polluted by automated `git commit -m "Updated coverage.svg"`;
Advantage: completely automated.
2. Drawback: need to be run manually on one's local machine and the files `coverage.xml` and `coverage-badge.svg` need to be commited;
Advantage: clicking on the badge should show an HTML report where one can visualize the testing summary and see which lines of code is tested and which isn't, like in https://github.com/openai/gym/issues/2734#issuecomment-1094573459 (the folder where the web report files are needs also to be commited).

Would anyone have any preference between those two solutions?