Update `test_thermodynamic_integration`

[PaulJonasJost]

adjusted test_thermodynamic_integration, by lowering the number of chains by factor of 5 and only running 10 chains, as the comparison of more chains is not something that we should have to test imo.

In local tests (Mac) it went from ~200sek to 20

[codecov-commenter]

Codecov Report

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

Project coverage is 84.48%. Comparing base (d5fde03) to head (14af5f2).

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1385      +/-   ##
===========================================
- Coverage    84.51%   84.48%   -0.03%     
===========================================
  Files          157      157              
  Lines        12956    12959       +3     
===========================================
- Hits         10950    10949       -1     
- Misses        2006     2010       +4     

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[arrjon]

True, this is not neceassary as a test. It was included to check convergence of the evidence estimation. Okay for me to remove it to speed up testing time.