Fix tests to work with mysql+postgres concurrently

Optimise tests.db.test_migrations and tests.db.test_migration_utils for
concurrency and make opportunistic tests obey walk_style configurations.

Closes-Bug: 1207750

Change-Id: Idb30f144fb67aefc58749897d774103c3c0ec640
Merge "Fix tests to work with mysql+postgres concurrently"