Add E2E tests

[dundalek]

Currently we test by either calling eval directly or spawning a special helper process.

It would be good to add tests that spawn the closh itself and send commands via stdin. This would also allow us to test UI interaction (e.g. history search).

There might be some trickiness to figure out to wait for the process to start and parsing out the prompt.

[dundalek]

Closing for now, will address it during readline reimplementation.