chore: using subTest to distinguish which method failed

Author: fatelei

Lib/test/test_bytes.py

@@ -2067,16 +2067,17 @@ def __init__(self, ba):
                 self.ba = ba
             def __index__(self):
                 self.ba.clear()
-                return 65
+                return ord("A")
 
         def make_case():
             ba = bytearray(b"A")
             return ba, Evil(ba)
 
         for name in ("find", "count", "index", "rindex", "rfind"):
             ba, evil = make_case()
-            with self.assertRaises(BufferError):
-                getattr(ba, name)(evil)
+            with self.subTest(name):
+                with self.assertRaises(BufferError):
+                    getattr(ba, name)(evil)
 
 
 class AssortedBytesTest(unittest.TestCase):