RED - Created classes, and added test for get children.
GREEN - getChildren() returns null (as expected), because no constructor
in TreeNode is implemented to initialize the ArrayList.
RED - Added test to check if children in TreeNode is initialized in its
constructor.
GREEN - ArrayList is now initialized in constructor.
REFACTOR - changed code to check if children is empty instead of null in
getChildrenTest().
RED - added tests for get/set parent
GREEN - get/set parent implemented
REFACTOR - changed name on the setParentTest-method.
RED - added test method for get data
GREEN - getData returns a TreeNode's data.
REFACTOR - Added @Before-method and removed all  TreeNode initiations
from test methods.