Retrieve unit test classes

I am trying to make a mojo that takes all the unit tests from all the modules that have a certain annotation made by me. The problem is that I can't access the unit tests from any module.
The pom structure looks like this:

root
|--ModuleA
|--ModuleB
|--ModuleC
|--ModuleD(the mojo)
`--pom.xml

The question is how to access or retrieve the unit test classes of each module when the mojo runs for it.