ci: Update run_tests.yml

Add the steps for setting up pytest and pytest-blender and running the tests inside of the "tests" directory

.github/workflows/run_tests.yml

@@ -11,3 +11,10 @@ jobs:
         with:
           blender-version: "4.2"
       - run: blender --version
+      - run: python -m pip install --upgrade pip
+      - run: |
+          pip install pytest-blender
+          python_blender_executable="$(pytest-blender --blender-executable $blender_executable)"
+          $python_blender_executable -m ensurepip
+          $python_blender_executable -m pip install pytest
+          pytest --blender-executable $python_blender_executable tests/