Fix test assumption that the CreateIterator of VariableBlockVector is a forward iterator

The CreateIterator of VariableBlockVector is currently an output iterator. This creates problems with the tests, which use std::fill() with those iterators, and that function actually requires forward iterators (because output iterators are not comparable). The libc++ implementation of clang 7 actually enforces this, causing test failures.

Looking at the implementation, making the iterator a forward iterator seems more trouble than it's worth, so fix the test.