add tests for yarn berry

[appsforartists]

Currently, wireit's test suite only covers yarn classic. This begins to include coverage for yarn berry.

This works in everything but Windows. Two months later, and we still haven't had the time/info to make it work in Windows. It's better to have some tests than no tests, so we're landing this for POSIX now, and tracking Windows testing in #1014.

[appsforartists]

@rictic @aomarks if either of you wanna take a look at this

[appsforartists]

I think this is the right direction.

We can use yarnPath to switch which version of yarn to use (which is the same that would happen in the wild). We can even encapsulate that in something like:

const getCommandForAgent(agent) => agent.startsWith('yarn') ? 'yarn' : agent

and switch the current for (const command of to for (const agent of. The only wrinkle is that we need to modify the filesystem of rig. IDK if there's an immutable way to do that, or where you'd wanna put that.