Minimal reproduction repository of an issue with including test files from one module to another.
There are two modules in the repository: submodule1 and submodule2. Submodule2 depends on submodule1. Dependent classes from submodule1 are visible in submodule2 without any issues. Same is for tests. So it means it is possible to create an instance of the class Example1 in the class Example2. Same is possible to do in Test2 with class Test1 from submodule1.
Now the issue come, when java modules are activated. To do so, rename module-info.java.txt to module-info.java. The compilation fail, because class Test1 become not visible in class Test2.