custom-set: update 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 defining a 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