Documentation - Test Binder configuration and Gradle dependency

[henriquels25]

In the Test Binder Configuration section, there are instructions to import the dependencies using maven and it says that it is easy to retrofit it into Gradle.

I had some problems trying to do this in Gradle, and even Spring Initilizr is not doing it right, as it can be seen in #1159 and #591.

I suggest adding instructions showing how to import the test dependency in Gradle because it is not as straightforward as other dependencies.

If it would be ok to do this, I'd be willing to create a PR.

[olegz]

Actually this has been resolved (documented) on the master with https://github.com/spring-cloud/spring-cloud-stream/pull/2055/files
I'll back port it to Horsham and resolve the isue

[olegz]

Back ported to 3.0.x branch 6f867ad