Add test to check all datamodel schemas

[braingram]

As came up https://github.com/spacetelescope/stdatamodels/pull/320#pullrequestreview-2290621622

It could be useful to do some basic schema checks for the datamodels schemas. These could include:

• ~a call to asdf.schema.check_schema~ EDIT: this is already handled by pytest-asdf
• checking if all schema keywords are in expected and understood places