Revert "Reduce GitHub Actions unit test durations by removing tox and…

… using test parallelization (#1345)"

This reverts commit 4663b33.

This appears to be incompatible with `v14` legacy tests; more investigation required.

.github/workflows/unittests.yaml

@@ -27,9 +27,7 @@ jobs:
           cache: pip
       - name: Install dependencies
         run: python -m pip install .[dev]
-      - name: Install parallel test runner
-        run: python -m pip install unittest-parallel
       - name: Run Tests
         env:
           PYTHONWARNINGS: "always:::recipe_scrapers,ignore:::recipe_scrapers.plugins.static_values"
-        run: unittest-parallel --level test
+        run: python -m unittest