The testing support in the CLI has proven 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.

As part of this change, the dependency management for Spock has been
moved into spring-boot-parent, thereby making it "private". This
allows it to continue to manage the test-only Spock dependency in
spring-boot-test without also managing the version of Spring that is
used by a user's application.

Closes gh-9087
Fixes gh-9043