Init integration tests

[Splines]

❗ This is based on #41, so #41 has to be merged into main beforehand.

How to add new test cases

• In whole_workflow_test, create a new method, e.g. eiffeltower(). Add it to the parameterized list of test_whole_workflow.
• In that method, call wiz.res(...) however you like.
• Create a file in tests/integration/fixtures/ that is named like the method (here: eiffeltower.tex).
• In that file, paste in the \newcommand*{...} you'd expect from the wiz.res(...) call.
• That's it, now run the tests. In VSCode, you will see the outputs from the test execution including the temporary results.tex file.