Fix method getGroupByName to use fullName of group

- return always only 1 group if exists and use fullName of group for
  searching
- add info to javadoc
- add regex for testing variable name of group in method getGroupByName
- add test for more than one groups with the same shortName but
  different Name
- fix formating of two existing tests (getGroupByNameWhenGroupNotExists
  and getGroupByNameWhenVoNotExists)