test[poisson]: Add missing multi-proc tests

[toutane]

The missing multiple processors (2 procs only for the GitHub CI) tests of PRs #165, #166, #167 and #168 are added.

Tests Test.L-shape.2D.nwcsr.arc, Test.L-shape.3D.nwcsr.arc and Test.sphere.3D.csr-gpu.arc were failing because of reference tests results update of PR #172, it is fixed now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.45%. Comparing base (d466211) to head (b134814).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #173      +/-   ##
==========================================
+ Coverage   63.39%   63.45%   +0.05%     
==========================================
  Files          57       57              
  Lines        8335     8335              
  Branches     1050     1050              
==========================================
+ Hits         5284     5289       +5     
  Misses       2776     2776              
+ Partials      275      270       -5     

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