+        self.assertDictEqual(self.conn._tunnel_headers, tunnel_headers)

You could just use assertEqual. Quoting from docs:

    "The list of type-specific methods automatically used by assertEqual() are summarized in the following table. Note that it’s usually not necessary to invoke these methods directly."

https://docs.python.org/3/library/unittest.html#unittest.TestCase.addTypeEqualityFunc

+        self.assertTrue(b'CONNECT destination.com' in self.conn.sock.data)
+        self.assertTrue(b'Host: destination.com' in self.conn.sock.data)

You could use assertIn instead.