tests: add testcase that shows that pyreverse will not extract the inheritance link for a flat folder as described in #7686

[Hannoma]

Type of Changes

	Type
✓	:bug: Bug fix

Description

As described in #7686, pyreverse does not extract the inheritance link for classes living in different files. In this PR I haved added the possibility to easily write testcases for pyreverse to generate a diagram for a complete directory / module. As far as I can tell this was not possible before just using the functional tests.

I also have a possible fix in mind for this issue, but am not sure, if I should add it in this PR or open another one. As @DudeNr33 suggested, I started with writing the test to reproduce the described issue.

Refs #7686