Enable dup() test with legacy FS. NFC (#23772)

Whatever bug existed with dup() seems to have now been fixed on the
legacy FS too.

Author: sbc100

test/test_other.py

@@ -13989,13 +13989,8 @@ def test_unistd_close_noderawfs(self):
     self.emcc_args += ['--pre-js', 'pre.js']
     self.do_run_in_out_file_test('unistd/close.c')
 
-  # WASMFS tests
-
-  # TODO: This test will only work with the new file system.
-  # Addresses this issue: https://github.com/emscripten-core/emscripten/issues/4017
-  # The new file system also correctly identifies errors that the JS file system missed.
-  def test_wasmfs_dup(self):
-    self.set_setting('WASMFS')
+  @also_with_wasmfs
+  def test_unistd_dup(self):
     self.do_run_in_out_file_test('wasmfs/wasmfs_dup.c')
 
   @also_with_wasmfs