tests: make flag parsing test more future safe

The revlog format contains a number of feature flags, e.g. if general
deltas are active. When testing that unknown flags are rejected, use
bits that are not immediately following the currently used bits.

Differential Revision: https://phab.mercurial-scm.org/D8782

tests/test-revlog-v2.t

@@ -32,10 +32,10 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') and None
+  ...     fh.write(b'\xff\x00\xde\xad') and None
 
   $ hg log
-  abort: unknown flags (0x04) in version 57005 revlog 00changelog.i!
+  abort: unknown flags (0xff00) in version 57005 revlog 00changelog.i!
   [255]
 
   $ cd ..