Fix workflow: add pytest-cov installation for coverage reports

Author: felixevers

.github/workflows/tests.yml

@@ -125,6 +125,11 @@ jobs:
           python -m pip install --upgrade pip
           pip install -r requirements.txt
 
+      - name: Install test dependencies
+        working-directory: backend
+        run: |
+          pip install pytest-cov
+
       - name: Run unit tests
         working-directory: backend
         run: |