Add tests

[Minoru]

I got a couple unit tests, but the project only has a few functions that can be unit-tested. The rest has to be end-to-end tested I guess.

I should find out how people test their microservices. I can probably borrow some ideas from there.

Some of the functionality depends on current time; e.g. an instance is "dead" if it had been failing checks for more than a "week". Should I run the app in a clock namespace so I can adjust its notion of time easily?