Add Unit Test for - Suffix Tree

File location : [Suffix tree](https://github.com/knaxus/problem-solving-javascript/blob/master/src/_DataStructures_/Trees/SuffixTree/index.js)
Hello folks, please go through the Contribution Guide here [https://github.com/knaxus/problem-solving-javascript#contribution-guide](https://github.com/knaxus/problem-solving-javascript#contribution-guide) before going forward with this issue. 

Thanks