Fix loading messages in tests for modules

As discovered in RESTWS-422 messages are not properly loaded when running tests in modules.

Our MutableResourceBundleMessageSource in core looks for messages in the following locations: /WEB-INF/messages, /WEB-INF/module_messages and /WEB-INF/custom_messages so it does not find messages on the test classpath. The tests may have been failing or passing depending on the environment.

This may need to be fixed in MutableResourceBundleMessageSource or in the test application context, by providing additional locations of messages.