Implement and use `Matcher#describeMismatch` throughout `axon-test` module

When using the Hamcrest Matcher based validation logic in ResultValidator, like expectEventsMatching(Matcher<? extends List<? super EventMessage<?>>> matcher), it is hard to see the actual mismatch, if something goes wrong.

This is due to the following implementation:

https://github.com/AxonFramework/AxonFramework/blob/e567af75fb44be19006533ba2aac1d0db614edd8/test/src/main/java/org/axonframework/test/aggregate/ResultValidatorImpl.java#L118-L129

It would be better to request the Matcher to describe the mismatch using the describeMismatch(Object item, Description description) method. This will result in a message that highlights the actual mismatch, not the whole matching logic.

I would suggest to change the implementation to:

    public ResultValidator<T> expectEventsMatching(Matcher<? extends List<? super EventMessage<?>>> matcher) {
        if (!matcher.matches(publishedEvents)) {
            final StringDescription description = descriptionOf(matcher);
            description.appendText("\nMismatch:\n");
            matcher.describeMismatch(item, description);
            reporter.reportWrongEvent(publishedEvents, descriptionOf(publishedEvents, matcher), actualException);
        }
        return this;
    }

    private StringDescription descriptionOf(Object item, Matcher<?> matcher) {
        StringDescription description = new StringDescription();
        matcher.describeMismatch(item, description);
        return description;
    }

[fernanfs]

I’ve created a PR, @smcvb. In this, I implemented the usage of the describeMismatch mechanic of Hamcrest Matchers. All changes should be backwards compatible. In the Reporter class, I’ve added a new method, that will receive the mismatch in addition to the description. The old method is not used anywhere in the codebase, thus I’ve deprected that method. Thus, it should be possible to accept the PR and still maintain backwards compatibility.

[fernanfs]

Thank you for your reply, @smcvb! I’m absolutely aware, that the number of changes to be made, are quite substantial, to have it consistent. Especially when trying to main a backwards compatible API within the custom matcher implementations. I’m currently giving that a shot, but do not count on me delivering a PR for the whole story anytime soon.

But what I can provide is a pull request that will only address the expectEventsMatching implementation by adding the output of describeMismatch as an additional information. This would not affect the backwards compatibility. For the Matcher-Implementations provided by axon, this still wouldn’t yield any substantial improvements. But when using Matchers implemented by Hamcrest (or custom ones) that support the describeMismatch, this would massively improve the output quality.