NavItem tests and to prop update

[stuller]

• "to" wasn't defined, so not sure how that was even working before. updated code in NavItem to use this.props.to instead, and added to to propTypes (I tried adding it up in the props list but that caused other issues, conflicts with react-router-dom I think caused errors on docs)
• added snapshot test
• added test file - this didn't add a lot of coverage, but it did add some. I think a lot was getting covered in Nav but I felt like it needed its own test file. There is one branch that I couldn't figure out, but everything else should have complete coverage now.