Limit maximum multiprocessing pool size in xeb tests

[pavoljuhas]

Problem: multiprocessing.Pool() uses all available cores which can cause problems on hardware with many cores.

Solution: Limit the maximum pool size to 4.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.83%. Comparing base (6037f18) to head (2b98779). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6713 +/- ## ========================================== - Coverage 97.83% 97.83% -0.01% ========================================== Files 1077 1077 Lines 92482 92484 +2 ========================================== + Hits 90480 90481 +1 - Misses 2002 2003 +1 ```

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