Tighten up the test-machine specs

[cddr]

In #230 we added helper functions for creating test-commands which helps in cases where the commands are authored using these new functions. @AndreaCrotti commented that it would be nice if we checked that the commands passed to run-test did indeed match the specs. This PR includes specs for some of the legacy commands left out of that PR so that we can now properly assert the test-command sequences passed to run-test.

To enable spec-checking behaviour during development evaluate (s/check-asserts true) at the REPL. When this is enabled and you attempt to run-test with an invalid command sequence, you'll get an exception like so

Checklist

• [x] tests
• [x] updated CHANGELOG (the "unreleased" section)