Added parent pointer tree skeleton tests

Author: FamALouiz

pydatastructs/trees/tests/test_m_ary_trees.py

@@ -1,5 +1,9 @@
 from pydatastructs import MAryTree
+from pydatastructs.trees.m_ary_trees import ParentPointerTree
 
 def test_MAryTree():
     m = MAryTree(1, 1)
     assert str(m) == '[(1, 1)]'
+
+def test_ParentPointerTree(): 
+    pass