Fail by default if a springboot() dependency graph contains junit or mockito.

[plaird]

That's it, I have seen one too many springboot() apps include junit or mockito by mistake. These are test deps, and don't belong in springboot apps. We have one counter example internally, but that is an exception.

DoD:

• Set by default: deps_banned = [ "junit", "mockito", ],
• Update the comment to be more clear on how to suppress the error if the check fails, to help beleaguered build engineers at 2am when the build fails
• Update the stardoc