Make tests folder non-optional

[teddygroves]

This make going through the wizard a little simpler

Checklist:

• [x] No pytest errors
• [x] make analysis works
• [x] README.md up to date
• [x] docs up to date
• [x] {{cookiecutter.repo_name}}/README.md up to date
• [x] investigate.ipynb up to date

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (fb4b90d) 100.00% compared to head (c74b818) 100.00%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #71 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 3 3 Lines 64 64 ========================================= Hits 64 64 ```

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