feat(modules_testdata_base_path): update guidelines for test data

[maxulysse]

@netlify /docs/contributing/test_data_guidelines

[netlify[bot]]

✅ Deploy Preview for nf-core ready!

Name	Link
🔨 Latest commit	907adc003bffcf930e796d5c6a0763187f0c5c7c
🔍 Latest deploy log	https://app.netlify.com/sites/nf-core/deploys/662b6992d800b6000814b023
😎 Deploy Preview	https://deploy-preview-2474--nf-core.netlify.app/docs/contributing/test_data_guidelines
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.