JIRA: https://jira.spring.io/browse/INT-799
Some test classes (e.g. `TestUtils`) were duplicated in core to avoid cyclic
dependency.
Now that core messaging has been moved to spring-messaging, it is possible
to remove the dependencies on `spring-integration-core` from `spring-integration-test`.
A few minor test cases have been moved to `spring-integration-core`.
The simple polishing to the `build.gradle` and `ServiceActivatorOnMockitoMockTests`
d16cd9748c