Fix test_fast when there's only one CPU

[tjni]

I ran into this when running the tests in a VM that only had one CPU.

[SaidBySolo]

Rather than an if statement, it would be better to write one more test function when there is only one worker.

I checked again and it seems to be ok :)

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.354% 🎉

Comparison is base (d255d1a) 88.429% compared to head (402aa43) 88.783%.

❗ Current head 402aa43 differs from pull request most recent head 1f6daea. Consider uploading reports for the commit 1f6daea to get more accurate results

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #2801       +/-   ##
=============================================
+ Coverage   88.429%   88.783%   +0.354%     
=============================================
  Files           92        92               
  Lines         7156      7052      -104     
  Branches      1228      1197       -31     
=============================================
- Hits          6328      6261       -67     
+ Misses         574       542       -32     
+ Partials       254       249        -5     

see 57 files with indirect coverage changes

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