Fix test_shadow_tables() on PostgreSQL/MySQL

Migration 209 creates a few new tables for storing of temporary
data (they all have names starting with 'dump_'). This breaks
test_shadow_tables() of ArchiveTestCase, which checks that every
existing table has a corresponding shadow table.

The DB API tests currently succeed only because they are run on
SQLite, and migration 209 is no-op for SQLite DB backend.

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

Change-Id: I632454fdacf35be972668f394496466978b5616f
Merge "Fix test_shadow_tables() on PostgreSQL/MySQL"