Consolidate AT/ITs for stream-applications

[onobc]

We currently have AT/ITs scattered about in a few places. Let's consolidate them into a form that makes sense and avoid duplication. Some of the places:

• https://github.com/spring-cloud/stream-applications-acceptance-tests
• https://github.com/spring-cloud/stream-applications/blob/main/applications/stream-applications-integration-tests/README.adoc
• Some overlap of testing functionality in SCDF ATs.

Originally posted by @onobc in #264 (comment)