bpo-40834: Fix truncate when sending str object with channel (GH-20555)

(cherry picked from commit 29c117202e386bad1d66ae336e2fefa1a1809ee0)

Co-authored-by: An Long <aisk@users.noreply.github.com>
diff --git a/Lib/test/test__xxsubinterpreters.py b/Lib/test/test__xxsubinterpreters.py
index e17bfde..7aec021 100644
--- a/Lib/test/test__xxsubinterpreters.py
+++ b/Lib/test/test__xxsubinterpreters.py
@@ -378,6 +378,9 @@
         self._assert_values(i.to_bytes(2, 'little', signed=True)
                             for i in range(-1, 258))
 
+    def test_strs(self):
+        self._assert_values(['hello world', '你好世界', ''])
+
     def test_int(self):
         self._assert_values(itertools.chain(range(-1, 258),
                                             [sys.maxsize, -sys.maxsize - 1]))