Append comparison diff to the error messages when applicable

This PR enhances the message output for comparison failures by adding also the diff part of message.

Instead of having the message just `Failed asserting that two strings are equal.`, it will be:

```
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'foo'
+'bar'
```

What pretty much clones the behavior of jUnit allure adapter output.