Change sqitch_db default to $db_name\_sqitch.

So by default there will be one sqitch metadata database for every
Sqitch-managed database. I think this will be less confusing in general (I ran
into it when trying to deploy the tutorial project to multiple databases in a
single MySQL instance). As a Postgres user I think of them as more like
schemas, but for MySQL users, it think they are really more like databases, so
let's just go with it.
Fix failing test broken in 16323c36.