py3: suppress the output of .write() calls in tests/test-revlog-v2.t

This makes the test pass on Python 3.

Differential Revision: https://phab.mercurial-scm.org/D3785

tests/test-revlog-v2.t

@@ -31,7 +31,7 b' Can create and open repo with revlog v2 '
 Unknown flags to revlog are rejected
 
   >>> with open('.hg/store/00changelog.i', 'wb') as fh:
-  ...     fh.write(b'\x00\x04\xde\xad')
+  ...     fh.write(b'\x00\x04\xde\xad') and None
 
   $ hg log
   abort: unknown flags (0x04) in version 57005 revlog 00changelog.i!