Add comments about what the test does

Author: GeorgianaElena

binderhub/tests/test_app.py

@@ -17,17 +17,21 @@ def test_help_all():
     check_output([sys.executable, '-m', 'binderhub', '--help-all'])
 
 def test_repo_providers():
+    # Check that repo_providers property is validated by traitlets.validate
+
     b = BinderHub()
 
     class Provider(RepoProvider):
         pass
 
+    # Setting providers that inherit from 'RepoProvider` should be allowed
     b.repo_providers = dict(gh=GitHubRepoProvider, gl=GitLabRepoProvider)
     b.repo_providers = dict(p=Provider)
 
     class BadProvider():
         pass
 
+    # Setting providers that don't inherit from 'RepoProvider` should raise an error
     wrong_repo_providers = [GitHubRepoProvider, {}, 'GitHub', BadProvider]
     for repo_providers in wrong_repo_providers:
         with pytest.raises(TraitError):