=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)
 

akka-docs-dev/rst/scala/code/docs/stream/FlowGraphDocSpec.scala

@@ -141,8 +141,9 @@ class FlowGraphDocSpec extends AkkaSpec {
         outlets: immutable.Seq[Outlet[_]]) = {
         assert(inlets.size == this.inlets.size)
         assert(outlets.size == this.outlets.size)
-        // This is why order matters when overriding inlets and outlets
-        PriorityWorkerPoolShape(inlets(0), inlets(1), outlets(0))
+        // This is why order matters when overriding inlets and outlets.
+        // The "[Nothing, Any]" is equivalent to casting the Inlets/Outlets.
+        PriorityWorkerPoolShape[Nothing, Any](inlets(0), inlets(1), outlets(0))
       }
     }
     //#flow-graph-components-shape

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
   }
 
   {