Add stage description and cache dicrectories

.gitlab-ci.yml

 image: node:12.16.3
 
+stages:
+  - test
+  - build
+  - deploy
+
 variables:
   MONGODB_VERSION: '4.2.6'
   MONGODB_URL: 'mongodb://mongodb:27017/test'
@@ -9,15 +14,18 @@ variables:
 # http://docs.gitlab.com/ee/ci/yaml/README.html#cache
 cache:
   paths:
-    - node_modules/
+    - app/client/node_modules/
+    - app/server/node_modules/
 
 test_client:
+  stage: test
   script:
     - cd app/client
     - npm install
     - npm run test
     
 test_server:
+  stage: test
   # Info: http://docs.gitlab.com/ee/ci/docker/using_docker_images.html#what-is-a-service
   services:
     - name: mongo:$MONGODB_VERSION