Add tests for `validate_schema_map` (#391) r=rnewman
Add tests for `validate_schema_map` (#391) r=rnewman

* Add tests for `validate_schema_map`

* Update test to ensure we get the right error out

ncalexan/mentat,mozilla/mentat,ncalexan/mentat,ncalexan/datomish,ncalexan/mentat,mozilla/mentat,mozilla/mentat,mozilla/mentat,ncalexan/mentat,ncalexan/mentat,mozilla/mentat,ncalexan/mentat,ncalexan/datomish,mozilla/mentat