It's t3st (gitlab.com: kstr0k/t3st -- can't post links I think), a shell testing library that I'm working on. It aims for... infinite scripting flexibility, or something. There are self-tests (in t/). The tests are shell scripts that produce TAP (using the library), and can run either via their own shebang, or via prove's "-e" option.
It currently runs under a lot of shells (multiple bash versions, zsh emulations etc), and it's not a biggie because the tests are simple; however I'd like to offer users (or at least myself in other projects) the option to parallelize different shells for potentially complex tests.