Use pytest fixture for multiprocessing Pool in z_phase_calibration_test

[pavoljuhas]

Creation of a default all-CPU multiprocessing Pool causes problems in
our internal test framework. Here we create the pool explicitly with
at most 4 new jobs.

No change in the effective test code.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.37%. Comparing base (bc6dab9) to head (5fab7bd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7713      +/-   ##
==========================================
- Coverage   99.38%   99.37%   -0.01%     
==========================================
  Files        1091     1091              
  Lines       97815    97821       +6     
==========================================
  Hits        97214    97214              
- Misses        601      607       +6     

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