add yarn check to ensure package.json doesn't diverge from yarn.lock

.gitlab-ci.yml

@@ -109,7 +109,8 @@ setup-test-env:
   script:
     - node --version
     - yarn --version
-    - yarn install
+    - yarn install --pure-lockfile
+    - yarn check # ensure that yarn.lock matches package.json
     - bundle exec rake gitlab:assets:compile
     - bundle exec ruby -Ispec -e 'require "spec_helper" ; TestEnv.init'
   artifacts: