Explore acceptance testing for SCION Infra servers (and maybe other apps)

[scrye]

The goal is to make acceptance testing easy to do for SCION applications, s.t. tests can be quickly added/expanded in the future.

The following should be considered and included in a document describing our acceptance testing methods:

• How to express the test (e.g., inputs, expected outputs)
• How to describe the tested system (the systems might be very different; some tests might only need to run a single app, while others need topologies with multiple routers and services)
• How to drive the test (e.g., bash scripts maybe?)
• How to run the tested system (e.g., docker might be involved)
• How to collect data about test results (success/failure, debugging artifacts)
• How to integrate in CI (if deemed desirable)

While this shares a lot with our current integration tests, we probably need a bit more flexibility for the above.

This is related to #1814 and #1846.