Flat container adaptors require the iterators of underlying containers
to be random access, and it is required that random access container
iterators must support three-way comparison ([container.reqmts]/39 - /41).
As a result, we should at least avoid testing "containers" with random
access but not three-way comparable iterators for flat container
adaptors.
This patch adds a new class template `three_way_random_access_iterator`
to `test_iterators.h` and fixes some usages of `MinSequenceContainer`
with the new iterators.
2f54a843bb