Introduce tests for the migration service.

[saikonen]

Suggested Improvement
Introduce tests for the migration service that cover basic acceptance behaviour:

• Migrations can be applied, and rolled back successfully (no broken migrations exist)
• Migration service can be introduced after-the-fact to an existing metadata service deployment (No conflicts with migrations with CREATE TABLE that the metadata service has applied)

Motivation
Currently introducing any migrations to the service relies heavily on manual testing and is prone to introducing unexpected issues. We also have the existing issue that the migration service cannot be applied successfully against an existing schema that was created by the metadata service. The migrations should preferably be somewhat idempotent (opting for CREATE IF NOT EXISTS) so the service can be an easy opt-in choice later down the line.