This is the first in a series of changes designed to solve unit testing

problems in RBTools. Traditionally, unit testing for anything using

rbtools.api (directly or indirectly) has been very difficult, as most

operations require making numerous API calls to compute such state as

API capabilities or repository lists. While we had a couple of helpers

throughout the codebase, they still left most of the work to the

particular unit test.
This change introduces rbtools.testing.api, which is where API-related

testing infrastructure will soon live, and a new

ResourcePayloadFactory class. This is responsible for building:


Object payloads/headers/URLs/etc. for items within a resource tree

   (such as a repository).


Response payloads (for lists/items/errors) representing what would

   be sent to a client.


Going forward, anything that needs to test against part of the API tree

should use this (or suitable wrappers, coming in the next change) to

build up the responses for the API resource classes to consume. This

gives us consistency in what sort of payloads we're testing against.
It's basically equivalent to the sort of testing methods we have

available in Review Board for creating instances of models with

defaults, such as review requests or repositories.
This is currently limited in the types of object payloads/metadata that

can be created. It should evolve over time as testing needs evolve.
Upcoming changes will make use of this to provide a more simple,

consistent mechanism for writing real-world unit tests that perform API

requsts against these payloads.

Tested along with some upcoming changes. Unit tests utilizing the

payloads in this class all succeeded in looking up and getting the

information they needed.