[3.13] pythongh-131234: Improve test_popen with more asserts (pythonGH-131235) (python#131240)

pythongh-131234: Improve `test_popen` with more asserts (pythonGH-131235)
(cherry picked from commit fc07f86)

Co-authored-by: sobolevn <mail@sobolevn.me>

Author: miss-islington

Lib/test/test_popen.py

@@ -57,14 +57,21 @@ def test_return_code(self):
     def test_contextmanager(self):
         with os.popen("echo hello") as f:
             self.assertEqual(f.read(), "hello\n")
+            self.assertFalse(f.closed)
+        self.assertTrue(f.closed)
 
     def test_iterating(self):
         with os.popen("echo hello") as f:
             self.assertEqual(list(f), ["hello\n"])
+            self.assertFalse(f.closed)
+        self.assertTrue(f.closed)
 
     def test_keywords(self):
-        with os.popen(cmd="exit 0", mode="w", buffering=-1):
-            pass
+        with os.popen(cmd="echo hello", mode="r", buffering=-1) as f:
+            self.assertEqual(f.read(), "hello\n")
+            self.assertFalse(f.closed)
+        self.assertTrue(f.closed)
+
 
 if __name__ == "__main__":
     unittest.main()