=doc  #16459 change confusing FaultHandlingDocSpec in Fault Tolerance Document

Code `FaultHandlingDocSpec` in section `Test Application` of document [Fault Tolerance](http://doc.akka.io/docs/akka/2.3.7/scala/fault-tolerance.html) does not work outside Akka project, as `AkkaSpec` class is not released along with akka-testkit library.
This could confuse new akka users who read the documentation.

Please `sphinx:generate` after merge.
This system should be called `"FaultHandlingDocSpec"`.