"ShellSpec is a full-featured BDD unit testing framework for dash, bash, ksh, zsh and all POSIX shells that provides first-class features such as code coverage, mocking, parameterized test, parallel execution and more."
Excerpt from https://shellspec.info/

In my experience, ShellSpec is more complete and easy to use than BATS.
And ShellSpec supports POSIX shells and not only bash.

I don't know if the project goals includes support for POSIX shells, but anyway I think is a good idea to keep this option open.