Currently for very small projects I use the built in NodeJS test tooling.
But for larger and more complex projects, I tend to use Vitest these days. At 40MBs down, and most of the dependency weight falling to Vite (33MBs and something I likely already have installed directly), it's not too heavy of a dependency.
It is based on vite and a bundler has no place in my backend. Vite is based on roll-up, roll-up uses some other things such as swc. I want to use typescript projects and npm workspaces which vite doesn't seem to care about.