User keyword teardown missing from running model JSON schema

[pekkaklarck]

The running model got JSON serialization support in RF 6.1 (#3902) and the JSON format got a schema. The schema currently doesn't include teardown with user keywords.

The reason our tests didn't notice this issue is that the schema allows extra attributes. Thus the teardown included in actual spec files was accepted although it wasn't in the schema. My intention was to not allow extra attributes in the schema, but apparently configuring the model root element wasn't enough and instead the configuration needs to be done in a common base class. This needs to be fixed as well.