I wasn't too worried about this earlier because the cache mainly stores
a bunch of numbers, but there's a little more than that: the generated
URLs also include original attachment filenames, which might be sensitive
(referencing people's names, bank names, etc.). Tighten security
accordingly.
This returns a cached version of the new rtutil.RT class,
so that a single process can easily grab references to one
instance with one cache of ticket/attachment data, etc.
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.
