We recently released multi-wallet support for the Yoroi Shelley Testnet extension! In the video below, Sebastien Guillemot provides additional details.
Download the extension here 
Chrome: https://chrome.google.com/webstore/detail/yoroi-shelley-testnet/bioklcnnnpdblghplkifbemcigeanmjn
Firefox: https://addons.mozilla.org/en-US/firefox/addon/yoroi-shelley-testnet/