Add Github Actions workflow and unit test for 'util.validate_setup_file()' and cli

[duylethanh]

This PR contains the following:

• Github Actions workflow triggered by pull requests on the master branch
• unit tests (with pytest) for util.validate_setup_file(), simex e launch --launch-through=fork and simex e purge --all -f
• ~add pytest6.1.2 to install_requires in setup.py~

The workflow currently uses the cross product of [ubuntu-latest, macos-latest] and [python3.6, python3.7] to run the unit tests The following screenshot is from a test pull request on my fork https://github.com/duylethanh/simexpal/pull/4:

Some things to consider:

• ubuntu-latest resolves to Ubuntu 18.04, although Ubuntu 20.04 is available as preview
• macos-latest resolves to macOS 10.15 (Catalina), although macOS 11.0 (BigSur) is available as preview
• currently the workflow takes roughly 30s-1min to run
• (for the future) in case the number of dependencies increases, we should consider caching
• I have stated a specific version of pytest, we might wanna relax that condition

[duylethanh]

PR updated. I made the requested changes. ~, while doing some refactoring to the new code. I also removed unused import statements.~

Are there any other simple but useful tests that we could add? Maybe running experiments via the Python API (= a fork launcher)? I guess we would have to copy the experiments directory to a temporary location for that to work.

I have added some cli tests via subprocess.check_call(). As we only verify the return code with this method we might run into similar problems like the following; simex e launch will launch the experiments or skip experiments if the instance is not available. Both actions will not cause an error and the program ends with return code 0.

This means, that we might want to parse the print() outputs of a subprocess and compare it to the expected output, to avoid different outcomes with the same return code.

edit: I removed the unrelated commits to keep this PR clean. I will make a separate PR for the minor fixes.