Allow to add plugin dependency to test classpath

[jansorg]

Describe the need of your request

In a project the tests, and only the tests, are using the JSON plugin.
I'd like to have the classes of the JSON plugin in the test classpath. I don't want them in the main classpath because I could use them unintentionally.

Proposed solution

Add a flag or parameter to plugin(), bundledPlugin(), bundledModule() ... to add the dependency into the test classpath and not into the production classpath

Alternatives you've considered

No response

Additional context

I'm using the v2 plugin descriptor, but this probably doesn't matter.

[Undin]

Actually, it's the same as #1586