libstdc++: Always return a sentinel<I> from __gnu_test::test_range::end()

libstdc++: Always return a sentinel<I> from __gnu_test::test_range::end()

It seems that in practice std::sentinel_for<I, I> is always true, and so the
test_range container doesn't help us detect bugs in ranges code in which we
wrongly assume that a sentinel can be manipulated like an iterator.  Make the
test_range range more strict by having end() unconditionally return a
sentinel<I>, and adjust some tests accordingly.

libstdc++-v3/ChangeLog:

	* testsuite/24_iterators/range_operations/distance.cc: Do not assume
	test_range::end() returns the same type as test_range::begin().
	* testsuite/24_iterators/range_operations/next.cc: Likewise.
	* testsuite/24_iterators/range_operations/prev.cc: Likewise.
	* testsuite/util/testsuite_iterators.h (__gnu_test::test_range::end):
	Always return a sentinel<I>.