net: add testnet seeds (for browser)

bcoin browser mode can not use DNS to discover seeds, so they are hard-coded.
see: https://github.com/bcoin-org/bdns/blob/master/lib/dns-browser.js#L23
There's already plenty in `/lib/net/seeds/main.js` so this is just to get browser-nodes running on testnet.
Seeds were pulled from my own `hosts.json` file on a live testnet bcoin node