Introduce rootCause() condition for use with the EngineTestKit

[sbrannen]

Overview

TestExecutionResultConditions currently provides a cause() condition that matches if an exception's cause matches all supplied conditions.

However, unless I've overlooked something, there is currently no easy way to create a condition that matches if an exception's root cause matches all supplied conditions.

While writing tests for the Spring Framework, I had a need for a rootCause() condition and implemented that in a new EngineTestKitUtils utility class.

Deliverables

• Introduce rootCause() condition in TestExecutionResultConditions.
• Document in Release Notes.