libstdc++: Add missing equality comparison in new tests [PR117921]

These new tests fail in Debug Mode because the allocator types aren't
equality comparable.

libstdc++-v3/ChangeLog:

	PR libstdc++/117921
	* testsuite/23_containers/set/modifiers/swap/adl.cc: Add
	equality comparison for Allocator.
	* testsuite/23_containers/unordered_set/modifiers/swap-2.cc:
	Likewise.

libstdc++-v3/testsuite/23_containers/set/modifiers/swap/adl.cc

@@ -36,6 +36,7 @@ namespace adl
       using propagate_on_container_swap = std::true_type;
 
       friend void swap(Allocator&, Allocator&) { swapped = true; }
+      friend bool operator==(Allocator, Allocator) { return true; }
     };
 }
 

libstdc++-v3/testsuite/23_containers/unordered_set/modifiers/swap-2.cc

@@ -43,6 +43,7 @@ namespace adl
       using propagate_on_container_swap = std::true_type;
 
       friend void swap(Allocator&, Allocator&) { swapped = true; }
+      friend bool operator==(Allocator, Allocator) { return true; }
     };
 }