Fix ApplicationHelper specs

There were several specs that were failing when run by themselves.

- Use the `helper` object, as per RSpec 3 standards
- Use `assign` to assign instance variables that helpers expect
- Add `StubConfiguration` support module

spec/spec_helper.rb

@@ -16,6 +16,7 @@ RSpec.configure do |config|
   config.include Devise::TestHelpers, type: :controller
   config.include LoginHelpers,        type: :feature
   config.include LoginHelpers,        type: :request
+  config.include StubConfiguration
   config.include TestEnv
 
   config.infer_spec_type_from_file_location!

spec/support/stub_configuration.rb

+module StubConfiguration
+  def stub_application_setting(messages)
+    allow(Gitlab::CurrentSettings.current_application_settings).
+      to receive_messages(messages)
+  end
+
+  def stub_config_setting(messages)
+    allow(Gitlab.config.gitlab).to receive_messages(messages)
+  end
+
+  def stub_gravatar_setting(messages)
+    allow(Gitlab.config.gravatar).to receive_messages(messages)
+  end
+end