This cleans up the existing command testing suite, making use of the new

CommandTestsMixin. This allows for more consistency in test setup and

execution, and will make it easier for us to start building new unit

tests as we update command code.
The tests no longer need the internal command-creation functions, most

of which were too specific to the existing tests and set state that

didn't make sense for the given command (due to copy/pastes of other

similar setup functions).
This also ends up fixing up some tests, depending on the testing

environment. The rbt alias tests were picking up user-owned settings

by default, but the new command infrastructure avoids this issue through

a more controlled home environment.

Command unit tests pass on all supported versions of Python.