Fix petsc unit test

[WeilerP]

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Description

Update unit test tests/test_linear_solver.py::TestLinearSolverPETSc::test_create_petsc_matrix_no_a_matrix.

Closes

Closes #1275.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.58%. Comparing base (2f34707) to head (c8e9521).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1276      +/-   ##
==========================================
+ Coverage   79.38%   80.58%   +1.20%     
==========================================
  Files          51       51              
  Lines        8618     8618              
  Branches     1491     1491              
==========================================
+ Hits         6841     6945     +104     
+ Misses       1198     1092     -106     
- Partials      579      581       +2     

see 3 files with indirect coverage changes

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