To validate Schema Registry's working correctly, a test script is deployed alongside the Client Testing when you run
make setup-client-test
In /tmp/kafkatest on your master, you'll find the following file
/tmp/kafkatest/<kafka_cluster_identifier>-schema-registry-testing.sh
That shell script's configured to point at the correct instance of Schema Registry and automates steps from https://docs.confluent.io/current/schema-registry/docs/intro.html#quickstart.
Exectuting it runs through basic actions to prove Schema Registry is working correcly and provides the expected output. If it looks good, it looks good.