add node flag to ava, since we are using modules

[mosch]

This was required for me while following your guide. Maybe it's a valuable addition for others?

[mrbbot]

Hey! 👋 Thanks for this, that's really helpful. 🙂 I'm going to hold off merging this though, until v2 is merged into master, which should be in the next 1/2 weeks, just to make Git-stuff easier. If you want, you could change the target from master to v2, and I'll merge it now. The AVA recipe is now over here: https://github.com/cloudflare/miniflare/blob/v2/docs/src/content/testing/ava.md.