Update Dockerfile.test

ufff i hope this works

backend/tests/Dockerfile.test

@@ -8,7 +8,7 @@ WORKDIR /app
 COPY . /app
 
 # Install dependencies (including coverage)
-RUN pip install --no-cache-dir -r requirements.txt coverage
+RUN pip install --no-cache-dir -r requirements.txt && pip install --no-cache-dir coverage
 
 # Run Django tests with coverage
 CMD ["sh", "-c", "coverage run manage.py test"]