Since I was asked on IRC, an example of converting an existing test. It's quite trivial really:

diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -630,9 +630,10 @@ class UTF16BETest(ReadTest, unittest.Tes
             (b'\xdc\x00\x00A', '\ufffdA'),
         ]
         for raw, expected in tests:
-            self.assertRaises(UnicodeDecodeError, codecs.utf_16_be_decode,
-                              raw, 'strict', True)
-            self.assertEqual(raw.decode('utf-16be', 'replace'), expected)
+            with self.subTest(raw=raw, expected=expected):
+                self.assertRaises(UnicodeDecodeError, codecs.utf_16_be_decode,
+                                  raw, 'strict', True)
+                self.assertEqual(raw.decode('utf-16be', 'replace'), expected)
 
     def test_nonbmp(self):
         self.assertEqual("\U00010203".encode(self.encoding),