tests: port test-cbor.py to Python 3

I suspect this b''.join() was a remnant of an earlier iteration of
this code, as it was building a string from a string.

Differential Revision: https://phab.mercurial-scm.org/D3505

contrib/python3-whitelist

@@ -36,6 +36,7 b' test-bundle2-multiple-changegroups.t'
 test-cappedreader.py
 test-casecollision.t
 test-cat.t
+test-cbor.py
 test-censor.t
 test-changelog-exec.t
 test-check-commit.t

tests/test-cbor.py

@@ -69,7 +69,7 b' class BytestringTests(unittest.TestCase)'
 
         dest = b''.join(cborutil.streamencodeindefinitebytestring(
             source, chunksize=42))
-        self.assertEqual(cbor.loads(dest), b''.join(source))
+        self.assertEqual(cbor.loads(dest), source)
 
     def testreadtoiter(self):
         source = io.BytesIO(b'\x5f\x44\xaa\xbb\xcc\xdd\x43\xee\xff\x99\xff')