CI: Replace mix test and mix coveralls with just mix coveralls

`mix coveralls` runs the tests by itself, exits with failure if one of them
fails and accepts the same args, so there is no reason to run two of
them at the same time.

.gitlab-ci.yml

@@ -52,8 +52,7 @@ unit-testing:
     - mix deps.get
     - mix ecto.create
     - mix ecto.migrate
-    - mix test --trace --preload-modules
-    - mix coveralls
+    - mix coveralls --trace --preload-modules
 
 unit-testing-rum:
   stage: test