py3: add b'' prefix in test/test-fileset.t

# skip-blame because just a b'' prefix

Differential Revision: https://phab.mercurial-scm.org/D3605

tests/test-fileset.t

@@ -434,7 +434,7 b" Test detection of unintentional 'matchct"
   > from mercurial import registrar
   > 
   > filesetpredicate = registrar.filesetpredicate()
-  > @filesetpredicate('existingcaller()', callexisting=False)
+  > @filesetpredicate(b'existingcaller()', callexisting=False)
   > def existingcaller(mctx, x):
   >     # this 'mctx.existing()' invocation is unintentional
   >     return [f for f in mctx.existing()]