Fix parallel tests in PyOP2 (#3925)

* python -m pytest causing issues

* oversubscribe cores

.github/workflows/pyop2.yml

@@ -27,7 +27,7 @@ jobs:
         shell: bash
         run: |
           sudo apt update
-          sudo apt install build-essential mpich libmpich-dev \
+          sudo apt install build-essential libopenmpi-dev \
             libblas-dev liblapack-dev gfortran libhwloc-dev libfabric-dev
 
       - name: Set correct Python version
@@ -120,7 +120,7 @@ jobs:
         working-directory: firedrake
         run: |
           source ../venv/bin/activate
-          # Running parallel test cases separately works around a bug in pytest-mpi
-          pytest -k "not parallel" --tb=native --timeout=480 --timeout-method=thread -o faulthandler_timeout=540 -v tests/pyop2
-          mpiexec -n 3 pytest -k "parallel[3]" --tb=native --timeout=480 --timeout-method=thread -o faulthandler_timeout=540 -v tests/pyop2
+          pytest -m "not parallel" --tb=native --timeout=480 --timeout-method=thread -o faulthandler_timeout=540 -v tests/pyop2
+          mpiexec -n 2 --oversubscribe pytest -m "parallel[2]" --tb=native --timeout=480 --timeout-method=thread -o faulthandler_timeout=540 -v tests/pyop2
+          mpiexec -n 3 --oversubscribe pytest -m "parallel[3]" --tb=native --timeout=480 --timeout-method=thread -o faulthandler_timeout=540 -v tests/pyop2
         timeout-minutes: 10