[ci] Use truncation strategy for migration tests

this is necessary when Mysql DDE statements (create/drop/truncate table) are used
because these are not supported by transaction strategy otherwise it will raise an
'ActiveRecord::StatementInvalid: Mysql2::Error: SAVEPOINT active_record_1 does not exist: ROLLBACK TO SAVEPOINT active_record_1'
exception.

https://stackoverflow.com/questions/13161394/activerecord-error-savepoint-active-record-1-does-not-exist