fix: prevent 100% CPU usage in large-scale test suites

[wellwelwel]

Fixes #603.

• It's still possible to set concurrency to unlimited by manually setting it to 0.
• The concurrency limit also apply to the watch mode.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.85%. Comparing base (2c002c0) to head (05e174e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #606      +/-   ##
==========================================
- Coverage   97.94%   97.85%   -0.10%     
==========================================
  Files          26       26              
  Lines        2188     2189       +1     
  Branches      432      399      -33     
==========================================
- Hits         2143     2142       -1     
- Misses         39       41       +2     
  Partials        6        6              

Flag	Coverage Δ
linux	97.85% <100.00%> (-0.10%)	⬇️
osx	97.85% <100.00%> (-0.10%)	⬇️

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