Mark `client` fixture as ‘private’
Mark `client` fixture as ‘private’

No tests are now using the `client` fixture directly so we can rename
it.

Python convention is to use an `_underscore` for things which should be
considered semi private.

This should discourage people from writing new tests with these old
fixtures.

New tests should always use `client_request`.

Want to be logged in with a different user? Call
`client_request.login(user)` first.

Don’t want to be logged in? Call `client_request.logout()` first (most
of our tests need to be logged in).

Need an instance of `Response` object not an instance of
`BeautifulSoup`? Use `client_request.get_response` or
`client_request.post_response`.

Need to pass in a URL, not arguments to `url_for`? Use
`client_request.get_url(…)` or `client_request.post_url(…)`.

Need to pass in a URL and get a response back? Use
`client_request.get_response_from_url(…)` or
`client_request.post_response_from_url(…)`.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin