Add automated testing for each command

[lkatalin]

Use functionality matrix #2

The test should check that 1) the command does not fail, and 2) the result is properly formatted according to https://github.com/sigstore/protobuf-specs/blob/main/protos/sigstore_trustroot.proto

[kommendorkapten]

This is the most important one to start with IMHO. As there is lot of repetitive code in the /cmd/app directory. Break that out into pkg and then add test for that.