Add missing "is" assertions to various tests (#15254)

* Add missing "is" assertions to various tests

* Fixing save-card-with-empty-result-metadata-test by simply asserting on API response

* Fix more failing tests

* Switch row-type-agnostic-test back to strings for expected

* Modify check for convert-id-to-string to also include keys whose :base_type is :type/Number, in addition to :type/Integer to fix failures in Oracle and Snowflake