Add integration tests for examples

We currently have the app from the Getting Started guide implemented for all backends, but there are no tests to ensure they all behave the same.

The app implements several simple binaries to create/display/publish/delete posts. Writings some tests that interact with all the binaries and assert their output is good would be great (it doesn't even have to cover all failure cases).

I wrote the assert_cli crate for a similar use-case. There might be better crates for that now, though. Or actually, it might be possible to do that in a shell script. (Don't know how well that works on Windows, though.)