fix: Process return code

[koubaa]

There is no crash on shutdown for 251+, but there was still some tests that allowed subprocesses without exceptions to throw a nonzero result. I modified this to only be the case for <251.

Here's the pipeline run for 251: https://github.com/ansys/pymechanical/actions/runs/12652827509

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.46%. Comparing base (009b819) to head (817a656).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1029      +/-   ##
==========================================
+ Coverage   86.71%   87.46%   +0.75%     
==========================================
  Files          35       35              
  Lines        2266     2266              
==========================================
+ Hits         1965     1982      +17     
+ Misses        301      284      -17