Fix AggregateDBApiTestCase on PostreSQL and MySQL

Test methods of AggregateDBApiTestCase use hard-coded
instance ids values instead of ones returned by *_create()
DB API methods. This imposes the following restriction on
autoincrement behavior: it must always start with 1 (which
might not be true, if a DB is not recreated for each test
case).

Blueprint: db-api-tests-on-all-backends

Change-Id: I4712c5d91c22b3b531048340f3cc827ce5ac438d
Merge "Fix AggregateDBApiTestCase on PostreSQL and MySQL"