Update all command unit tests to use the new testing functionality.

Review Request #12248 — Created April 21, 2022 and submitted — Latest diff uploaded




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.