There is no sample/reference for usage of fragment in GraphQL query with respect to Content Fragments Schema in the official docs - https://experienceleague.adobe.com/docs/experience-manager-cloud-service/content/headless/graphql-ap...
I was able to arrive at this query by examining the Docs section in GraphiQL local IDE that the result of Query testList is TestModelResults. Hence created fragment out of the same.
fragment on TestModel (the one you did) will be able to output the OOTB/standard variables like _path/_typename not the custom/dynamic ones(headline)