Extend schema registry tests

Schema registry tests for publishing and consuming new schemas.

The tests only cover the api availability. Further reference should follow the official documentation as this is a quite extensive / complex subject, which would require code generation, plugins to have as example while the [official documentation](https://docs.confluent.io/platform/current/schema-registry/index.html) is quite good.

modules/redpanda/src/test/java/org/testcontainers/redpanda/RedpandaContainerTest.java

@@ -27,6 +27,7 @@
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.assertj.core.api.Assertions.tuple;
+import static org.hamcrest.Matchers.hasItems;
 
 public class RedpandaContainerTest {
 
@@ -77,9 +78,21 @@ public void testSchemaRegistry() {
                 // }
             );
 
-            io.restassured.response.Response subjects = RestAssured.given().when().get(subjectsEndpoint).andReturn();
-
-            assertThat(subjects.getStatusCode()).isEqualTo(200);
+            String subjectName = String.format("test-%s-value", UUID.randomUUID());
+
+            // register the new subject
+            RestAssured
+                .given()
+                .contentType("application/vnd.schemaregistry.v1+json")
+                .pathParam("subject", subjectName)
+                .body("{\"schema\": \"{\\\"type\\\": \\\"string\\\"}\"}")
+                .when()
+                .post(subjectsEndpoint + "/{subject}/versions")
+                .then()
+                .statusCode(200);
+
+            // list all the registered subjects
+            RestAssured.given().when().get(subjectsEndpoint).then().statusCode(200).body("$", hasItems(subjectName));
         }
     }