libstdc++: Make tests for std::ranges access functions more robust

libstdc++: Make tests for std::ranges access functions more robust

	* testsuite/std/ranges/access/end.cc: Do not assume test_range::end()
	returns the same type as test_range::begin(). Add comments.
	* testsuite/std/ranges/access/rbegin.cc: Likewise.
	* testsuite/std/ranges/access/rend.cc: Likewise.
	* testsuite/std/ranges/range.cc: Do not assume the sentinel for
	test_range is the same as its iterator type.
	* testsuite/util/testsuite_iterators.h (test_range::sentinel): Add
	operator- overloads to satisfy sized_sentinel_for when the iterator
	satisfies random_access_iterator.