In my opinion such mock-/helper-/util-classes which are exclusively required for testing should belong in a separate "test-jar" and should not be distributed with the normal Openfire installation.
Speaking for Maven you would generally create a separate "test-module", which contain these classes. This project would be built separately as "test-jar" and would be referenced by the plugins which need it.
See also here: http://stackoverflow.com/questions/174560/sharing-test-code-in-maven
So, my vote goes for "plugin SDK or similar separate project". Please not in the "main" src folder. Maybe a 3rd project which contains interfaces, which are shared between the core and the test project is needed too.
Do you really even care that much about unit testing, code coverage, and continuous integration for Openfire plugins?
It's certainly a good idea. As for now, you have to code "in the dark", build plugin jar, copy the jar to the Openfire installation and test it in the running system. Then repeat for every error you find.
Guus is very busy these days
Hm, there are many open issues, which require or ask for Guus' opinion/decision. Hopefully he'll find some time.