This introduces a new mixin for command unit tests,

rbtools.testing.commands.CommandTestsMixin. This offers two new

methods:


create_command(): Creates an instance of a command that can be

  initialized or inspected.


run_command(): Runs a command with provided input and arguments,

  returning the instance, exit code, and output/error content.


These methods can accept a pre-computed repository_info and tool,

which will be used instead of repository scanning, making it possible

to run a test against a real or simulated repository. If not provided,

scanning will still be disabled unless passing scan=True.
It also registers URLMapTransport as the transport for the command. A

setup_transport_func argument can be passed in to create_command()

to allow the tests to register URLs or set capabilities on the

transport.
There's an important fix included for unit tests that set

needs_temp_home = True. The home directory will be set as the current

directory, ensuring that the RBTools repository's own .reviewboardrc

won't interfere with tests. This is a better default behavior than

what we had, and will meet the expectations of most command unit tests.
Upcoming changes will make use of this new functionality.

Made use of this with some in-progress unit test additions, and with

a change that updates all existing command unit tests.