Write tests for multi-root trees

[fremail]

Here is the full list of functions:

• [x] createRoot
• [x] fetchRoot
• [x] fetchTree
• [x] fetchRoots
• [x] hasPrevSibling
• [x] hasNextSibling
• [x] hasChildren
• [x] hasParent
• [x] getPrevSibling
• [x] getNextSibling
• [x] getSiblings
• [x] getFirstChild
• [x] getLastChild
• [x] getChildren
• [x] getDescendants
• [x] getParent
• [x] getAncestors
• [x] getNumberChildren
• [x] getNumberDescendants
• [ ] insertAsParentOf
• [ ] insertAsPrevSiblingOf
• [ ] insertAsNextSiblingOf
• [ ] insertAsFirstChildOf
• [ ] insertAsLastChildOf
• [ ] moveBetweenTrees
• [ ] moveAsPrevSiblingOf
• [x] moveAsNextSiblingOf
• [ ] moveAsFirstChildOf
• [ ] moveAsLastChildOf
• [x] makeRoot
• [x] addChild
• [x] isLeaf
• [x] isRoot
• [x] isEqualTo
• [x] isDescendantOf
• [x] isDescendantOfOrEqualTo
• [x] isAncestorOf
• [x] isValidNode
• [x] detach
• [x] delete
• [ ] insertNode
• [ ] updateNode
• [ ] shiftRlValues
• [ ] shiftRlRange