libstdc++: Add testcase for fs::path constraint recursion [PR106201]

libstdc++-v3/ChangeLog:

	PR libstdc++/106201
	* testsuite/27_io/filesystem/iterators/106201.cc: New test.

libstdc++-v3/testsuite/27_io/filesystem/iterators/106201.cc

+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+// { dg-require-filesystem-ts "" }
+
+// PR libstdc++/106201 constraint recursion in path(Source const&) constructor.
+
+#include <filesystem>
+#include <ranges>
+using I = std::counted_iterator<std::filesystem::directory_iterator>;
+static_assert( std::swappable<I> );
+using R = std::counted_iterator<std::filesystem::recursive_directory_iterator>;
+static_assert( std::swappable<R> );