How can I get select messages ciphers for salsa20poly1305 for unit tests?
Just use the test vector specified by DJB himself, specified here, chapter 4 (just XSalsa20) and chapter 10 for the box. This will already test all your primitives.

To perform boundary tests, I would strongly suggest you first document the boundaries. These are the places where there is a difference in e.g. buffer handling for the MAC function. If you test those boundaries against your own library then the MAC will still generate the same wrong value.

There seems to be TweetNaCl.js available online, so you can generate your own vectors after establishing your boundaries.