gh-128377: Skip test_cmd_line.test_non_interactive_output_buffering when PYTHONUNBUFFERED=1 (#128378)

The `test_cmd_line.test_non_interactive_output_buffering` test assumes a buffered `stdio`
without checking the `PYTHONUNBUFFERED` value. Instead of changing the environment
variable for the duration of the test, it is better to simply skip it.

Author: WolframAlph

Lib/test/test_cmd_line.py

@@ -336,6 +336,8 @@ def test_osx_android_utf8(self):
         self.assertEqual(stdout, expected)
         self.assertEqual(p.returncode, 0)
 
+    @unittest.skipIf(os.environ.get("PYTHONUNBUFFERED", "0") != "0",
+                     "Python stdio buffering is disabled.")
     def test_non_interactive_output_buffering(self):
         code = textwrap.dedent("""
             import sys