custom-set: update, remove & re-order tests

updated tests for custom-set exercism#328

In the process of updating the tests I ended up simplifying(hopefully)
the tests cases. I removed all the extra methods and functions including
String which fundamentally changed the tests. I've also added a stub
with only the Set type in it, so that the exercise focuses on the set
operations rather than how to define the set type. This also means that
the set type is agreed up front and so there's no need to accomodate all
possible types that people could come up with for a set by defining it
with something like a string method.

Also relevant for reference:
exercism/problem-specifications#232
exercism/problem-specifications#257