Plugin authors: ":help dev-test-plugin"[1] describes how to use Nvim's own test framework in your repo/CI, using a small Makefile.
Further work is planned[2] where the end goal is to ship the test framework with Nvim itself.
[1] neovim.io/doc/user/dev_t…
[2] github.com/neovim/neovim/…
Posts refer to Nvim 0.13 (dev build).
Discuss usage/config/ideas at: github.com/neovim/neovim/…