Introduce a test suite for system testing

[haerdib]

Currently, the provided examples are used for system testing. But to have a full-suit system test, more tests (but less verbose) should be introduced. That way, more functionality could be tested, without replicating everything.

Current examples should still be tested in CI, to make sure they are up to date.

Open problem: How to provide (CI tested) examples that use the PlainTip? Nightly job with substrate node template (which is not provided as docker image currently)?