This loads settings from the same environment variables and ~/.rtrc
file as the rt CLI.
Note that it does *not* support RTCONFIG and the config file
searching, because right now that seems like more work for more
trouble to me.
As we start reading more sensitive user configuration (like RT credentials),
this fixture makes it simpler to ensure that the tests never read the user's
real configuration, which could lead to problems like passwords in tracebacks.