[libc++][test] Move iter_swap into iterator.cust.swap. NFC.

[jloser]

Summary

Move iter_swap.pass.cpp into a new subdirectory: iterator.cust.swap
for symmetry with the neighboring subdirectory iterator.cust.move.

[• Quuxplusone]

This moves files in the opposite direction from how I ordinarily want to; but in fact the reason we don't just git mv iterator.cust/iterator.cust.swap.pass.cpp iterator.cust/iter_swap.pass.cpp instead, is that the iterator.cust.move/ subdirectory actually contains three test files. So the subdirectory structure actually is serving some purpose in that case, and so getting this stuff onto a consistent subdirectory level sounds good.

libcxx/test/std/iterators/iterator.requirements/iterator.cust/iterator.cust.swap/iter_swap.pass.cpp

 // template<class I>
 // unspecified iter_swap;
 
 #include <iterator>
 
 #include <array>
 #include <cassert>
 
-#include "./unqualified_lookup_wrapper.h"
+#include "../unqualified_lookup_wrapper.h"
 #include "test_iterators.h"
 
 using IterSwapT = decltype(std::ranges::iter_swap);
 
 struct HasIterSwap {
   int &value_;
   explicit HasIterSwap(int &value) : value_(value) { assert(value == 0); }