libstdc++: Add a testsuite range type that has a sized sentinel

libstdc++: Add a testsuite range type that has a sized sentinel

This adds a testsuite range type whose end() is a sized sentinel to
<testsuite_iterators.h>, which will be used in the tests that verify LWG 3355.

libstdc++-v3/ChangeLog:

	* testsuite/util/testsuite_iterators.h (test_range::get_iterator): Make
	protected instead of private.
	(test_sized_range_sized_sent): New.