Migrate all tests to use the dict-based config format instead of hanging items off HomeserverConfig (#5171)

It doesn't really belong under rest/client/v1 any more.

The Mailer expects the config object to have `email_smtp_pass` and
`email_riot_base_url` attributes (and it won't by default, because the default
config impl doesn't set any of the attributes unless email_enable_notifs is
set).