move requirements_test.txt to tests directory

Author: craigbarratt

.github/workflows/validate.yml

@@ -30,5 +30,5 @@ jobs:
       - uses: "actions/setup-python@v1"
         with:
           python-version: "3.x"
-      - run: python3 -m pip install -r requirements_test.txt
+      - run: python3 -m pip install -r tests/requirements_test.txt
       - run: pytest --cov=custom_components