Complete test coverage of int_set

[rhalbersma]

• cover all cases for op== and op<
• add tests for for_each and reverse_for_each
• eliminate unreachable code in iterator nested class
• add tests for iterator post-increment and post-decrement
• add tests for iterator addressof
• add tests for iterator and initializer_list overloads of op=, insert and erase
• add test for capacity
• add test for swap
• add tests for intersects and disjoint