Add some form of integration testing

[jappeace]

For example on one thread you spinnup a keter app with some config,
and then on another you craft a tarbal that spins up a process that just echos a specific repsonse.

The test would then be on a particular port if keter sends back the appropriate response.

This is annoying because I'm not sure if this is allowed in a nix container (but we could find a workaround for that).

[jappeace]

This would be a single happy path test for loading an app in keter
eg `describe "happy path" $ it "loads a bundle" budleLoadTest
there are already some building examples in incoming
run keter on one thread, move in the bundle on another thread, and then wait till it's loaded, and see if it gives the right response
eg status code 200 with some response programmed in the app that's bundled

[jezen]

This project does indeed need a serious investment in a real test suite. I think the right way to go about this is with nixosTest, which would loosely involve building keter, running it in a VM, copying bundles to it, and exercising it by making requests with cURL. I intend to work on this at Thaiger Sprint along with other general maintenance (but, you know, no promises).

I'm not sure where the concern is with threads. Maybe I'll discover that when I work on it.