Add Unit Test For - Binary Tree preOrder Traversal

[TheSTL]

👽 What is required?

Function to test: preOrder();
The algorithm is here : Binary Tree

• Follow the ESLint rules strictly
• Follow the filename and folder structure strictly
• Since the Contribution Guide is not ready yet, go through the codebase pattern and add tests accordingly

Thank You!

[reshmisaji]

can I pick this up?

[TheSTL]

I'm closing this issue.
Merged PR - [#111 ]