=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/io/StreamFileDocSpec.scala

@@ -32,6 +32,7 @@ class StreamFileDocSpec extends AkkaSpec(UnboundedMailboxConfig) {
     //#file-source
     import akka.stream.io._
     //#file-source
+    Thread.sleep(0) // needs a statement here for valid syntax and to avoid "unused" warnings
   }
 
   {