=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/scala/code/docs/stream/FlowDocSpec.scala

@@ -199,7 +199,7 @@ class FlowDocSpec extends AkkaSpec {
     val (promise, cancellable, future) = r11.run()
 
     // Type inference works as expected
-    promise.success(0)
+    promise.success(())
     cancellable.cancel()
     future.map(_ + 3)