Replace mutable `config` default argument in FarcyTest.
Replace mutable `config` default argument in FarcyTest.

Default arguments in python should not be mutable types, as only one instance
of the object is created at parse-time, rather than a new instance each time a
default is required.

appfolio/farcy,balloob/farcy,balloob/farcy,balloob/farcy,appfolio/farcy,appfolio/farcy