ci: run tests with and without MongoDB

.github/workflows/rust.yaml

@@ -32,16 +32,23 @@ jobs:
         with:
           command: test
 
-      - name: Copy .env.example
-        if: github.event_name != 'pull_request'
-        run: |
-          cp .env.example .env
-
       - name: Run services in background
         if: github.event_name != 'pull_request'
         run: |
           docker-compose -f docker-compose.db.yml up -d
 
+      - name: Run cargo test (with MongoDB)
+        uses: actions-rs/cargo@v1
+        env:
+          MONGODB: mongodb://localhost
+        with:
+          command: test
+
+      - name: Copy .env.example
+        if: github.event_name != 'pull_request'
+        run: |
+          cp .env.example .env
+
       - name: Start API in background
         if: github.event_name != 'pull_request'
         run: |