Define test dependencies separately

Previously, the test dependencies were installed every time the action was ran, even though they are not used.

Author: per1234

.github/workflows/libraries_report-size-deltas.yml

@@ -27,7 +27,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install --requirement "$GITHUB_WORKSPACE/libraries/report-size-deltas/requirements.txt"
+          pip install --requirement "$GITHUB_WORKSPACE/libraries/report-size-deltas/tests/requirements.txt"
 
       - name: Lint with flake8
         run: |