docs(test): add instruction on how to integrate test in pre-commit hook

Created by: luftywiranda13

Since Jest added a flag called --findRelatedTests, some users asked about how to do this in CRA.

I've helped to answer them but I think it's a good time to document this now in Running Tests section. So then we can guide them to the docs.

close #2587 (closed) #2336 (closed)