chore/doc: move all tests to namespace to help Doxygen

This helps in two ways:
1. Doxygen is confused if there are multiple class defintions with the
   same name, which happens frequently in tests, e.g. "ModuleA"
2. Doxygen shows rather prominently all classes defined in the tests
   when viewing the Class List, while the important classes are "hidden"
   at first below the ChimeraTK namespace.