Add Unit Test For Find unique-word-count in Trie

### 👽 What is required?
Add unit test to find unique word count in a trie.
The algorithm is here : [unique-word-count](https://github.com/knaxus/problem-solving-javascript/tree/master/src/_DataStructures_/Trees/Trie/unique-word-count)

Contribution guide is here : [CONTRIBUTION](https://github.com/knaxus/problem-solving-javascript/blob/master/CONTRIBUTING.md)

Thank You!