bpo-44022: Improve the regression test. (pythonGH-26503) (pythonGH-26508)

It wasn't actually detecting the regression due to the
assertion being too lenient.
(cherry picked from commit e60ab84)

Co-authored-by: Gregory P. Smith <greg@krypto.org>

Author: miss-islington

Lib/test/test_httplib.py

@@ -977,7 +977,12 @@ def test_overflowing_header_limit_after_100(self):
             'r\n' * 32768
         )
         resp = client.HTTPResponse(FakeSocket(body))
-        self.assertRaises(client.HTTPException, resp.begin)
+        with self.assertRaises(client.HTTPException) as cm:
+            resp.begin()
+        # We must assert more because other reasonable errors that we
+        # do not want can also be HTTPException derived.
+        self.assertIn('got more than ', str(cm.exception))
+        self.assertIn('headers', str(cm.exception))
 
     def test_overflowing_chunked_line(self):
         body = (