workflows: Add mpiexec -n 1 to serial tests

Author: ZoeLeibowitz

.github/workflows/pytest-petsc.yml

@@ -74,7 +74,7 @@ jobs:
 
     - name: Test with pytest - serial
       run: |
-        ${{ env.RUN_CMD }} pytest -m "not parallel" --cov --cov-config=.coveragerc --cov-report=xml ${{ env.TESTS }}
+        ${{ env.RUN_CMD }} mpiexec -n 1 pytest -m "not parallel" --cov --cov-config=.coveragerc --cov-report=xml ${{ env.TESTS }}
 
     - name: Test with pytest - parallel
       run: |