Stunner - refactor integration tests

Some modules have tests which adds unwanted dependencies to the module and make it less independent from other modules. See example.

It happens due to integration nature of some tests, to test integration properly we need to use some addition dependencies and it makes whole architecture complicated and not clear.

Task for this issue:

• Identify integration tests across the kie-wb-common-stunner module
  • Like Integration Marshaller tests, forms generation, upcoming Selenium tests
• Move common test logic (as Marshaller initiators) to separate module as src
• Move integration tests under test folder for created module
• Remove unused dependencies after clean up.