tests: fix command name in test-blackbox.t to be bytes

This command is only used when tests are run with --with-chg, so this was missed
before.

Differential Revision: https://phab.mercurial-scm.org/D7553

tests/test-blackbox.t

@@ -390,7 +390,7 b' when using chg, blackbox.log should get '
   > from mercurial import registrar, scmutil
   > cmdtable = {}
   > command = registrar.command(cmdtable)
-  > @command('noop')
+  > @command(b'noop')
   > def noop(ui, repo):
   >     pass
   > EOF