title: Play Framework test helpers need implicit `Materializer`
body: <p>I'm using Play 2.6.x and the test helper for <code>status(result)</code> has the method:<br>
  <code>def status(of: Accumulator[ByteString, Result])(implicit timeout: Timeout, mat: Materializer): Int = status(of.run())
  </code></p>
  
  <p>Running tests throws when the compiler can't find the implicit value:
  <code>could not find implicit value for parameter mat: akka.stream.Materializer</code></p>
  
  <p>What is the Materializer -- I'm assuming it's part of Akka-HTTP</p>
  
  <p>And how can I provide one?</p>