=str,htp clean up build warnings

- explicitly provide Unit values and place parens around tuple creation
- remove structural type usage in TestUtils
- fix Java double-casts
- use unused Java values by asserting their non-nullness
- work around inability to place case class in trait (scripted test)

The remaining warnings about using private types in public methods are
bogus as reported in https://issues.scala-lang.org/browse/SI-9490.

akka-docs-dev/rst/java/code/docs/http/javadsl/HttpClientExampleDocTest.java

@@ -16,6 +16,7 @@ import akka.http.javadsl.model.*;
 import akka.http.javadsl.Http;
 import scala.util.Try;
 
+@SuppressWarnings("unused")
 public class HttpClientExampleDocTest {
 
     // compile only test