Test a few magics

[illright]

Here are a couple of unit tests for magics using pytest. Some of these tests are not passing yet, the relevant issues are #12946, #12947, #12948, #12949.

This PR also attempts to lay down some structure for the tests of magics by separating the tests for each magic into a class, grouping these classes into a module. This structure also allows for precise placement of test fixtures which are a very nice way of organizing setup/teardown in pytest.

Feedback would be highly appreciated, as we are new to this project and its structure.
For one, I don't think that we are able to get a fully working testing environment just by running pip install -e .[test]: we would still be missing matplotlib and pytest which we would have to install manually. I've brought up this issue in #12924 and got very comprehensive responses, but this aspect is still left unanswered. How to manage dependencies in this project?

[illright]

This is currently a work-in-progress PR (forgot to mark it as one, and now I can't seem to find the button).

[ivanov]

This is currently a work-in-progress PR (forgot to mark it as one, and now I can't seem to find the button).

Converted this for you - a link to convert to draft was under the "Reviewers" section.

[illright]

By the way, the checks are failing because pytest tests are being run with iptest, therefore, the fixtures don't work.

[illright]

Alright, we pushed our tests, now we could use some help with setting up the CI properly. Also I think the pytest tests are currently not being included in coverage.

[illright]

There's one test that is failing for reasons unknown to us. It is IPython.core.tests.test_magics.test_history.TestRerun.test_no_args