ataru can handle multiple files and test methods are named after md files

- Now, ataru can handle multiple files:

```bash
$ ataru my_file.md my_file_2.md
```

- Test method names contain the filename of the markdown file.
Is it then a good practice to always wrap have methods in classes and no global methods?
We noticed we handle methods differently in those two cases, but we were thinking with @kazjote to do the opposite - make `add_test_cases` a global method.