=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-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala

@@ -339,7 +339,6 @@ private[akka] object ActorProcessorFactory {
       case Scan(z, f, _)              ⇒ interp(fusing.Scan(z, f, settings.supervisionDecider))
       case Fold(z, f, _)              ⇒ interp(fusing.Fold(z, f, settings.supervisionDecider))
       case Recover(pf, _)             ⇒ interp(fusing.Recover(pf))
-      case Scan(z, f, _)              ⇒ interp(fusing.Scan(z, f, settings.supervisionDecider))
       case Expand(s, f, _)            ⇒ interp(fusing.Expand(s, f))
       case Conflate(s, f, _)          ⇒ interp(fusing.Conflate(s, f, settings.supervisionDecider))
       case Buffer(n, s, _)            ⇒ interp(fusing.Buffer(n, s))