Improve test reliability by cleaning the state before running test_remove_missing_module

[sturmianseq]

This PR aims to improve reliability of the test test_remove_missing_module by cleaning the state before running it.

The test can fail on the second run by running it twice: pip3 install pytest-repeat; python3 -m pytest --count=2 tests/test_operator.py::Test_operator::test_remove_missing_module.

    def test_remove_missing_module(self):
        with pytest.raises(AttributeError):
>           vlx.remove_module(cli_module)
E           Failed: DID NOT RAISE <class 'AttributeError'>

More specifically, this PR cleans the pre-state by ensuring that the cli_module is not in vlx.modules. In this way, the test does not fail on the second run any more.

[DrTexx]

Ah excellent, thank you @sturmianseq !
I'd missed that, I appreciate the PR.

Merged! :)