Support for custom message resolver

[Kleeven81]

In our project we are using a custom message resolver which we would also like to be able to use in the testing framework. Currently for each test a new TestEngineMessageResolver is created with no way to replace it with a custom implementation.

[Kleeven81]

A probable easy fix would be to add a private field for an IMessageResolver with getter and setter methods. During initialisation of the TemplateEngine in execute(final ITestable testable, final TestExecutionContext context) the field could be checked and if not null the custom IMessageResolver should be used instead of the standard TestEngineMessageResolver.