libstdc++: Qualify functions used in tests

libstdc++: Qualify functions used in tests

These tests rely on ADL for some functions, probably unintentionally.
The calls only work because the iterator wrappers derive from
std::iterator and so namespace std is an associated namespace.

libstdc++-v3/ChangeLog:

	* testsuite/25_algorithms/inplace_merge/constrained.cc: Qualify
	call to ranges::next.
	* testsuite/25_algorithms/is_sorted/constrained.cc: Likewise.
	* testsuite/25_algorithms/is_sorted_until/constrained.cc:
	Likewise.
	* testsuite/25_algorithms/swap_ranges/1.cc: Qualify call to
	swap_ranges.