In order to allow testing unhappy path as proposed in #1941 I suggest that we implement script testing based on github.com/rogpeppe/go-internal/testscript. This package was chosen because it is the only well maintained and extensively used script testing package available with the features that we need. Alternatives such as JS, CEL, starlark, or yaml are not chosen because they are either too strong, too weak or impose a learning cost that is higher than should be expected for a testing system.

Closes #2868