Thank you for the PR. I am currently pondering where to move spix in terms of general architecture and am leaning towards keeping the test server and command interface in Spix as minimalistic as possible.
Commands that build on top of the existing, more elemental, ones would then not be part of spix itself, but rather go into an additional library.

The advantage of this would be that it makes it easier to write alternative TestServers, as they do not have to be extended whenever a command is added. Especially if those TestServers were to be written by 3rd-parties.

I recently started adding architecture docs and planned an update to the library architecture here: #129

So, for now i would prefer to leave this PR on hold until the architecture changes are done and then decide what the best approach is.