move away from bunny CDN for the stress test

[mmomtchev]

• cleaner and more readable cache expiry, especially when recovering from HTTP errors
• do not use bunny CDN for the stress tests, the corruption is not reproducible without it

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 64.28571% with 15 lines in your changes missing coverage. Please review.

Project coverage is 74.69%. Comparing base (d9deb15) to head (13bf050). Report is 41 commits behind head on main.

Files	Patch %	Lines
src/vfs-http-worker.ts	72.22%	10 Missing :warning:
src/vfs-sync-http.ts	25.00%	3 Missing :warning:
src/vfs-http.ts	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #179 +/- ## ========================================== - Coverage 74.88% 74.69% -0.19% ========================================== Files 8 8 Lines 1967 1988 +21 Branches 204 204 ========================================== + Hits 1473 1485 +12 - Misses 487 496 +9 Partials 7 7 ```

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