add spec and fixture for rust nested test modules

[dyllandry]

Make sure these boxes are checked before submitting your pull request:

• Add fixtures and spec when implementing or updating a test runner
• Update the README accordingly
  • Didn't, I don't think necessary.
• Update the Vim documentation in doc/test.txt
  • Didn't, I don't think necessary.

The added failing spec demonstrates the issue #708

vim-test isn't running the correct test command for rust files, with both cargo test and nextest, when tests in a single file are nested inside modules. The linked issue has an example.

[dyllandry]

Some context from #708, I don't know much vimscript, but it was suggested I open a PR that adds a spec and fixture to demonstrate the problem. I managed to do that. Let me know if I can improve it.

Hopefully someone else who knows more about vim-test than I can fix the underlying issue. If no one comes by for long enough that I've learnt enough vimscript, I'll try fixing it.

[codeinabox]

Thank you for doing this. I am trying to figure out issues with the tests running in the pipeline and then can look into this later, though this looks great so far