#11 Provide enough test coverage for bug with unique calls

[Sergey S. Lyalin]

See related bug #2870186.
Enough test coverage for different cases for that bug should be provided. Consider both cases: 1) when there is no copying between creation of non-constant iterator and modification of the target object and 2) when there is a copying between those two points. Category (2) is almost not covered in test base.