Fix dependency issue with cmd2-ext-test plugin #1517

Now that we put out a 3.0.0b1 beta release for cmd2, much of our automatic CI/CD is failing due to unsatisfiable requirements related to the cmd2-ext-test plugin.

Regardless of the path forward, the current situation we have where testing cmd2 requiring a plugin which itself depends on cmd2 is an abomination of circular dependency that needs to end. A plugin which depends on cmd2 should not IMO be part of the main repository.

I see three general possible paths forward:

1. Deprecate the cmd2-ext-test plugin, delete the code for it, and include the mixin class in our isolated tests
2. Move the cmd2-ext-test plugin to a separate repository and modernize it so it works with cmd2 3.x
3. Bake the mixin class into cmd2 itself so it is available to all users and deprecate the plugin

One option is to document for users how they can create this mixin tests for their own testing ease.