Update sub_tree_test.cpp

Main algorithm: 
step1: flatten smaller tree to an array in pre-order traversal. 
step2: traverse pre-order (root first then left and right) the big tree and compare root nodes key value. 
step3: when matched, traverse big tree from current pre-order and compare one-by-one with the nodes in vector. 
Note: in this implementation I also flatten the bigger tree which can be generate huge vectors. Alternative, which may also be more efficient is to traverse both tree together and compare. Each has its own merits. 

Need to verify: whether this is correct when all nodes equals. 

Variant: What if both tree are BST?