New test util stuff for testing schema migrations (#13258)

backend/junit/test/metabase/junit.clj

modules/drivers/mongo/test/metabase/test/data/mongo.clj

...drivers/snowflake/test/metabase/driver/snowflake_test.clj

modules/drivers/sqlite/test/metabase/test/data/sqlite.clj

src/metabase/db.clj

src/metabase/db/liquibase.clj

test/metabase/db/schema_migrations_test.clj

test/metabase/db/schema_migrations_test/impl.clj

test/metabase/test/data/h2.clj

test/metabase/test/data/impl.clj

test/metabase/test/data/interface.clj

test/metabase/test/data/postgres.clj

test/metabase/test/data/sql.clj

test/metabase/test/data/sql/ddl.clj

test/metabase/test/data/sql_jdbc/execute.clj

test/metabase/test/data/sql_jdbc/load_data.clj