I'm getting to ready to extend this hook to deal with income receipts as
well as expense receipts. These changes let me write the tests in a more
declarative style, so I don't have to duplicate them all again to test
credits as well as debits.
Note that we're only testing debits right now, just like the existing tests,
because the default check_type for check() is PostType.DEBIT. Part of making
the changes will be changing that to PostType.BOTH.