* Consistently lock in ensureVersionTable and do not call ensureVersionTable from Drop across all database implementations
* Add test for dropping postgres databases
* Fix failing database tests
* Fix CockroachDb test, lock table should be created before versionTable
* Add Initialize() to Driver interface, and add integration tests for Drop() between database implementations and migrate
* Remove Initialize, document breaking behaviour of Drop
* Revert introduction of Initialize method
* Removed Initialize in Stub as well
* Remove call to non-existent Initialize and make sure to close re-initialized database connections
* Revert changes to TestDrop in database/testing
* Split Test and TestMigrate into different test entrypoints
* Remove unused import in migrate_testing
* Remove erroneous code to fix tests
* Add stub source imports to database tests
* Add Stub source to migrate tests
* Use example migrations for tests
* Add file driver to database tests
* Align database directory layout
* Add file source driver to Cassandra
* Review changes
* Minor syntactic change for cleaner diff
- Leaving migrate/testing in case there are unknown consumers
- Add migrate/dktesting package
- Update tests to use migrate/dktesting instead of migrate/testing
