Move a migration test to MigrationTestCase.

There is test_migration_get_unconfirmed_by_dest_compute
test in old mixed DbApiTestCase. This patch moves this test
to its native test case MigrationTestCase.

DbApiTestCase renamed as DecoratorTestCase because no
db api methods tests remained in this test case.

blueprint db-api-tests

Change-Id: Ibe6ebf266a2f4694fb218266ba644be825889907
Merge "Move a migration test to MigrationTestCase."