Add tests to dynamic cross modules example

[asarkar]

The dynamic cross modules example doesn't include any tests, so, the example doesn't work when tests are added to any of the modules. Please add some tests to one or more of the modules so that we have a mix of test and run targets, which is more practical and useful. This will, of course, also necessitate updating the build.sc file.

[asarkar]

I created a minimal example project to the best of my understanding how dynamic cross module tests can be discovered. I'm attaching it for reference. cross.zip