Our command testing support has been pretty under-utilized so far. While

working on some new tests, I encountered some crashes that could occur

if setting up two command runs in one test.
This change fixes that, along with some unbound variables. It also adds:

Full typing throughout the command test helpers
JSON output in the command results
Generics for the test helper class, to help type instances of the

  command classes

Upcoming tests will use this support.

Unit tests pass.
Tested with in-progress changes.