[3.14] gh-150922: Fix test_cmd libedit assertion in test_bang_completion_without_do_shell (GH-156623) (#157156)

gh-150922: Fix test_cmd libedit assertion in test_bang_completion_without_do_shell (GH-156623)

GNU readline retypes the full completed word after tab completion, but
libedit redraws only the untyped suffix following a cursor-forward
escape sequence. Assert on the common suffix ('ello') instead of the
literal 'hello' bytes so the test passes on both backends.
(cherry picked from commit 0b4c7da)

Co-authored-by: Shardul Deshpande <iamsharduld@users.noreply.github.com>

Author: miss-islington

Lib/test/test_cmd.py

@@ -316,7 +316,10 @@ def default(self, line):
         for input in [b"! h\t\n", b"!h\t\n"]:
             with self.subTest(input=input):
                 output = run_pty(script, input)
-                self.assertIn(b'hello', output)
+                # libedit redraws only the untyped suffix after a
+                # cursor-forward escape, unlike GNU readline which retypes
+                # the whole completed word, so check the common suffix.
+                self.assertIn(b'ello', output)
                 self.assertIn(b'tab completion success', output)
 
 def load_tests(loader, tests, pattern):