Add tests for PAYNT and payntbind

[TheGreatfpmK]

I would love to add tests and now it seems I will have some time to do so. Therefore I will create a branch called tests and I will start adding some tests there. Any help with this is welcomed!

I know testing PAYNT overall will be painful and therefore I will start with tests for some of the stuff in payntbind.

[TheGreatfpmK]

I created the branch where we can develop tests, and we will periodically merge it to master https://github.com/randriu/synthesis/tree/tests

I used pytest since StormPy already uses that and we can also use the Storm test models in our tests.

To run the tests simply execute pytest payntbind/tests while in the venv. For now I only added payntbind test setup.

[TheGreatfpmK]

I added the structure for PAYNT tests. One thing I would like to do is add the test step to our GitHub workflow.

[TheGreatfpmK]

I merged the tests branch to master and we will continue the development of tests in the master branch.

[TheGreatfpmK]

Added the tests to GitHub workflows, moreover, the workflows are now fixed and will be run in every branch with these tests (including PRs).