Remove testing support from the CLI

[wilkinsona]

The testing support in the CLI is proving to be more trouble than it's worth. Our recommendation is that, once an app gets to the stage of requiring a test suite, it should be converted to a Maven or Gradle project. This makes it easy to version, publish, deploy etc using the vast ecosystems of the two build systems. See gh-9043 for further background.

[StephenOTT]

@wilkinsona would be good if a note was added to the docs (cli docs) about the removal of this. There are many examples on the web that discuss the spring test command.

[snicoll]

@StephenOTT thanks for the ping. I've added a note in the migration guide as we don't document removed features in the doc in general. Please consider creating a new issue rather than commenting on a closed one next time.