Update .gitlab-ci.yml

.gitlab-ci.yml

 image: alpine:latest
 
+stages:
+  - test
+  - deploy
+
+test-job:
+  stage: test
+  script:
+    - echo "Testing files exist..."
+    - test -f  public/index.html
+    - echo "Test finished, all ok"
+  only:
+    - master
+
 pages:
   stage: deploy
   script:
-  - echo "Deploying to https://transfer.hft-stuttgart.de/pages/$CI_PROJECT_PATH/"
+    - echo "Deploying to https://transfer.hft-stuttgart.de/pages/$CI_PROJECT_PATH/"
   artifacts:
     paths:
-    - public
+      - public
   only:
-  - master
+    - master