Feature: App integration testing harness

[lann]

As a parallel to #352 it would be great to have a spin test subcommand that can run integration tests against a Spin app. Along with being a useful user feature, we would benefit from being able to easily CI against our examples/.

A minimal version of this might just spin up the app, run a test command configured in spin.toml, then SIGINT the app.

[lann]

cc @radu-matei