channels: Fix test for introductory commit signer.

channels: Fix test for introductory commit signer.

Until now the test would always succeed because it would return
'failed, which has truth value.  The exercise
'verify-introductory-commit', we need to add a second commit beyond the
introductory commit, which is what we do here.

* tests/channels.scm ("authenticate-channel, wrong first commit
signer"): Add a second commit and authenticate up to that one.  Change
'message?' to 'message-condition?'.