CI: run tests multi-threaded (#2251)

.github/workflows/unittests.yml

@@ -54,9 +54,9 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install pytest pytest-subtests
+        pip install pytest pytest-subtests pytest-xdist
         python ModuleUpdate.py --yes --force --append "WebHostLib/requirements.txt"
         python Launcher.py --update_settings  # make sure host.yaml exists for tests
     - name: Unittests
       run: |
-        pytest
+        pytest -n auto