fix: axios does not preserve symbols in its config

[susnux]

It seems Axios does not preserve entries in the request config which have a symbol-key.
So this refactors the code (see commit 3) to use string keys instead.

Commit 1 migrates the unit tests to proper tests without mocking axios.
So they are more like e2e tests (meaning we really also test integration with Axios).
Moreover I migrated them to use vitest browser mode as this library never runs in Node env but is intended for browsers.

Commit 2 only fixes the filesystem structure, we either use tests, __tests__ or next-to-code test files but never test folder.

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.30%. Comparing base (fe19736) to head (6a4c5c5).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/interceptors/not-logged-in.ts	60.00%	2 Missing ⚠️
lib/index.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #916      +/-   ##
==========================================
- Coverage   91.80%   87.30%   -4.51%     
==========================================
  Files           5        5              
  Lines          61       63       +2     
  Branches       19       19              
==========================================
- Hits           56       55       -1     
- Misses          5        8       +3     

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