gh-159191: Stress test bytes.join() with non-bytes items

The existing stress tests now only take the fast path. test_bytes is
part of the PGO training set, so the general path was barely trained,
and PGO builds stopped inlining PyBuffer_Release() there: joining a
list that is half bytearrays became about 14% slower than on main.
With this test it is back to main's speed.

Author: christianaurichzm

Lib/test/test_bytes.py

@@ -655,6 +655,8 @@ def test_join(self):
         seq = [b"abc"] * 100000
         expected = b"abc" + b".:abc" * 99999
         self.assertEqual(dot_join(seq), expected)
+        seq = [bytearray(b"abc")] * 100000
+        self.assertEqual(dot_join(seq), expected)
         # Stress test with empty separator
         seq = [b"abc"] * 100000
         expected = b"abc" * 100000