generate a submodule to run pankti-generated test cases

We may generate a submodule that contains all the pankti-generated test cases. The benefits of doing so include:

- A good separation between original test cases and pankti-generated
- Only `Test*PanktiGen.java` and `object-data.xml` need to be copied to the submodule, because this module depends on all the original submodules
- No need to modify original pom files, because only this module needs `xstream` as a dependency

This is an example that I manually did for TTorrent: https://github.com/gluckzhang/ttorrent/tree/ttorrent-2.0-pankti

What I did in this example:

- `mkdir pankti-tests` and add `pankti-tests` in the root `pom.xml`
- create a pom file for submodule `pankti-tests`, add all of the original submodules as its dependencies, add `xstream` as its dependency as well: https://github.com/gluckzhang/ttorrent/blob/ttorrent-2.0-pankti/pankti-tests/pom.xml
- put the pankti-generated test files in `src/test/java`, put the object data xml files in `src/test/resources`

```bash
cd path/to/pankti/pankti-generate/output/generated/ttorrent
find . -type f -name "Test*PanktiGen.java" -exec cp --parents {} path/to/ttorrent/pankti-tests/src/test/java \;
cp *.xml path/to/ttorrent/pankti-tests/src/test/resources
```

*This is for the convenience of our experiments. At last we might still put the generated test cases in the original submodules to create a PR.*

Use the TTorrent pankti-tests example as the reference, then inspect the repository root pom.xml and the existing module layout. Define a pankti-tests submodule with its own pom.xml, original submodules and xstream as dependencies, and place Test*PanktiGen.java files under src/test/java with object-data.xml under src/test/resources. Done means the generated tests can run from the new module without modifying the original module poms.

Written by the indexing model from the issue text.