Stunner - [DMN Modeler] Dynamic Form Properties with nested hierarchy. Additional test coverage.

Following from https://issues.jboss.org/browse/JBPM-7166 the PR containing the fix was merged for 7.14 however lacked sufficient test coverage to pass the normal (more stringent) review as there was Unit Test coverage and possibly some outstanding improvements to be made. See:-

https://github.com/kiegroup/kie-wb-common/pull/2241/files#diff-42d73d0a90151d64e2253ba9b4852358R124 (a "TODO")

https://github.com/kiegroup/kie-wb-common/pull/2241/files#diff-29408982b790c65730aae361d6a2501aR44 (a "TODO")

https://github.com/kiegroup/kie-wb-common/pull/2241/files#diff-b6807e2576042e3f3dddc606027864b9R63 (code formatting)

https://github.com/kiegroup/kie-wb-common/pull/2241/files#r230922780