wasi: add fs.FS integration test

[codefromthecrypt]

Right now, we mostly focus on unit tests with a little ad-hoc here and there wrt file systems. It should be possible to add a fs.FS impl that calls WASI. Doing this would allow us to re-use existing tests for parity, like iotest.TestReader, as well show use cases like root vs workdir file usage operates with less clutter.