Update dbtutils tests

[sydneynotthecity]

The dbtutils library introduced breaking changes during the major version upgrade from 0.9.5 to 1.1.1.

The recency test changed the expected data type from DATETIME to TIMESTAMP. This PR updates all dbtutils recency tests to reflect this change.

Updating these tests to closed_at, which is not the partition field on some tables, does not impact query performance or cost because these tests aggregate the field referenced:

select max(<timestamp>)
from `crypto-stellar.crypto_stellar.<table>`
..
..
..

In most cases, this query processes ~5GB or less, even for very large tables, like trust_lines.