Test all supported dialects

[jvatic]

Currently there's only a few unit tests, and they're focused on generic and PostgreSQL dialects. I'd like to have integration tests covering all supported dialects (see #9).

There's a few kinds of tests I'd like to have:

• List of migrations -> schema: Run migrations and generated schema against (separate) empty databases, assert they both have the same schema. (And do this in reverse when there are down migrations, for each one.)
• List of migrations + edited schema + expected migration: Generate migration from edited schema, assert the generated migration matches the expected one. (This can be a source for the previous one, using the expected migration.)
• Unit tests for diff that compare two schemas, and output a migration.
• Unit tests for migrate that apply a migration to a schema.