test: add tests for environment variables and runtime config overrides

[Hebilicious]

🔗 Linked issue

Extracted from #1318

closes #1150

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

While working on #1318 , I realised it was better to extract the tests from this PR into a standalone PR, as they can now be used to test useRuntimeConfig with every presets.
I noticed a bug in the cloudflare preset as well that I included here, however I can include it in another PR if that's better.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[codecov]

Codecov Report

Merging #1549 (2d71fb9) into main (e35e6b2) will decrease coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1549      +/-   ##
==========================================
- Coverage   76.54%   76.45%   -0.10%     
==========================================
  Files          76       76              
  Lines        7773     7773              
  Branches      783      781       -2     
==========================================
- Hits         5950     5943       -7     
- Misses       1821     1827       +6     
- Partials        2        3       +1     

see 2 files with indirect coverage changes

[pi0]

Thanks!