Hosting service test code is a bit of a mess, due to the need for

overriding HTTP request handling for most tests and setting up

account/service state. Over time, as code was copy/pasted between test

suites to form new tests, this just became more complicated and harder

to manage.
This change introduces a new infrastructure for writing hosting service

tests. Modern test cases now subclass

reviewboard.hostingsvcs.testing.HostingServiceTestCase, which, unlike

the old ServiceTests, is meant to be publicly-used. It provides

several public functions for common types of checks needed, some of

which were previously private functions in ServiceTests, and some of

which are new and will help slim down most tests.
There's a set of new attributes for specifying defaults for hosting

account data, repository extra_data, authentication credentials, and

more, which reduce the need to hard-code the same bits of data

everywhere.
It also introduces the new setup_http_test context manager, which does

all the work of setting up state and spies for any tests that need to

fake results from HTTP requests. This provides a couple of ways of

working. It allows for an explicit payload to be provided, or a status

code for an error response, or a function to invoke when the HTTP

request is made. The context manager yields a HttpTestContext object,

which has some pre-calculated state and helpers for creating

repositories for the test and asserting the results of the HTTP calls.
It also makes the useful HostingService.get_field public, which can

now be used in tests without being re-implemented.
No tests have been updated to make use of this yet. That will come in

future changes.

Existing unit tests pass.
Tested with some upcoming changes that will make use of the new test

case class.
Built the docs and made sure things rendered mostly fine (along with

some changes in beanbag-docutils that will go in later).