Correction of within time in TransformationSampleJapiSpec, see #2875

* We had the same issue in ticket 2820 and then the conclusion was that
  the within timeout should be increased due to "publish on convergence",
  but it was not done correctly in TransformationSampleJapiSpec, see
  https://github.com/akka/akka/commit/8bf7c03158cd4e064d3eecef1b331d14c2990775
* Also fixed the wrong (but harmless) subscribe in TransformationBackend
Merge pull request #1008 from akka/wip-2875-TransformationSampleJapiSpec-patriknw

Correction of within time in TransformationSampleJapiSpec, see #2875