htlcswitch: fix flake in TestShutdownIfChannelClean
htlcswitch: fix flake in TestShutdownIfChannelClean

When Alice receives a CommitSig near the end of the test, the test
assumed that she wouldn't reply with a RevokeAndAck message before
shutdownAssert was called. This led to a test flake. Instead, only
call shutdownAssert after we've received the RevokeAndAck.

aakselrod/lnd,aakselrod/lnd