Minor improvements to testsuite iterator utilities

Minor improvements to testsuite iterator utilities

	* testsuite/util/testsuite_iterators.h (BoundsContainer::size()): Add
	new member function.
	(WritableObject::operator=): Constrain with enable_if when available.
	(remove_cv): Use std::remove_if when available.
	(test_container::it(int)): Use size().
	(test_container::size()): Use BoundsContainer::size().

From-SVN: r277578