Apply the same GC fix to DirSingletonTest#test_for_fd

test_for_fd has the same pattern as test_fchdir: the Dir from
`Dir.new(Dir.pwd).fileno` is collectable before `Dir.for_fd(fd)` runs,
so an ill-timed GC closes the fd and the call fails with Errno::EBADF.
Keep the Dir alive in a local and close it in ensure.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hsbt

test/stdlib/Dir_test.rb

@@ -133,14 +133,17 @@ def test_foreach
   end
 
   def test_for_fd
-    fd = Dir.new(Dir.pwd).fileno
+    dir = Dir.new(Dir.pwd)
+    fd = dir.fileno
 
     with_int(fd) do |int|
       assert_send_type(
         "(::int) -> ::Dir",
         Dir, :for_fd, int
       )
     end
+  ensure
+    dir&.close
   end
 
   def test_getwd