We've had two prior attempts at creating a usable API transport for unit

tests. The first, MockTransport, simply does nothing. It's an empty

stub of a transport that could be subclassed. The second,

TestTransport, could hardcode one list and one root payload, but was

limited to that.
This implements a new transport that replaces these two. This one

provides a formal way of constructing as much or as little of an API

tree as needed for tests. It supports registering combinations of URLs

and HTTP methods to response information (payloads, headers, and HTTP

status codes).
By default, a standard root resource, API info resource, and repository

list resource is registered. We may want to add other defaults in the

future, but these are required for most API client usage.
Unit tests can call add_item_url() to add an item (or singleton)

resource, add_list_url() to add a list resource, or add_url() to add

a more generic response payload.
URLs can be modified after registration, allowing a unit test to change

aspects of the root resource or any other registered resource based on

their needs.
They can also manipulate the capabilities attribute to change API

capabilities as needed.
Handling is strict, so any malformed or missing URLs will trigger

assertion errors.
To use the transport, URLMapTransport can be passed in as the

transport class to RBClient or to Command. Upcoming changes will

make it easier to integrate this with Command for unit testing.

Tested along with upcoming changes to Command unit testing, and

build a handful of unit tests for another upcoming change. All

tests pass.