How can I get select messages ciphers for salsa20poly1305 for unit tests? In order to avoid memory errors, I always write safe C++ wrappers with strong type-safety for any C encryption library I use.

To test my wrapper of crypto_secretbox_xsalsa20poly1305 of libsodium and make sure I'm using it correctly, I need a set of messages, nonces and keys that someone used and encrypted and the outcome of that. This is necessary for unit tests.

In OpenSSL, when I do that, I usually use the terminal, and pipe from the terminal to my unit tests and compare the output of my result.

There doesn't seem to be a terminal tool for libsodium. So I'm hoping there's either an online service that does this (take msg + nonce + key and produce cipher + mac), or someone here could provide a few test
messages and their expected mac + cipher in hex.
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.