Increased concurrency

[PatrickHaecker]

This reduces test time by increasing concurrency.
This is mostly still at the vibe-coding state as I have not gone through all the changes. I want to avoid that this gets lost, so I create a draft PR, but it probably makes sense to first finish #389/#390 – first make it work (correctly), then make it fast.

[codecov]

Codecov Report

❌ Patch coverage is 96.18321% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.12%. Comparing base (7249b88) to head (0b40bbf).

Files with missing lines	Patch %	Lines
src/persistent_tasks.jl	95.45%	4 Missing ⚠️
src/ambiguities.jl	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #391      +/-   ##
==========================================
- Coverage   87.80%   87.12%   -0.69%     
==========================================
  Files          12       12              
  Lines         525      598      +73     
==========================================
+ Hits          461      521      +60     
- Misses         64       77      +13     

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