Revert "disable pytest-xdist (to check CI failure)" (#6127)

* Revert "disable pytest-xdist (to check CI failure) (#6077)"

This reverts commit 5e8de55.

* Apply suggestions from code review

* Apply suggestions from code review

Author: mathause

.github/workflows/ci-additional.yaml

@@ -96,7 +96,7 @@ jobs:
           python -c "import xarray"
       - name: Run tests
         run: |
-          python -m pytest \
+          python -m pytest -n 4 \
              --cov=xarray \
              --cov-report=xml \
              $PYTEST_EXTRA_FLAGS

.github/workflows/ci.yaml

@@ -87,7 +87,7 @@ jobs:
         run: |
           python -c "import xarray"
       - name: Run tests
-        run: python -m pytest
+        run: python -m pytest -n 4
           --cov=xarray
           --cov-report=xml
           --junitxml=pytest.xml