refactor greylisting tests

we do not need to check for a DUNNO on second try in all tests
better check once, but then actually for both results:
 - first retry too early, defer again
 - second retry OK

this should trigger fails for SQLite as described in #16
always use `datetime.datetime.now()`

different databases are not sure how current CURRENT_TIMESTAMP is
as we are always comparing with a loca `now()` anyways, let's use
it for the database too.

closes #16