add a sanity check ensuring that Numba threading actually works

[slayoo]

[codecov[bot]]

Codecov Report

:x: Patch coverage is 47.82609% with 12 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 86.45%. Comparing base (64877ed) to head (7d3c47d). :warning: Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
PySDM/backends/numba.py	47.82%	12 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1690      +/-   ##
==========================================
- Coverage   86.50%   86.45%   -0.06%     
==========================================
  Files         412      412              
  Lines       10372    10385      +13     
==========================================
+ Hits         8972     8978       +6     
- Misses       1400     1407       +7     

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[slayoo]

codecov marks is as uncovered because codecov is run with JIT compilation disabled, while the added logic is only tested with JIT compilation enable