Refactor DataStreamAutoShardingServiceTests

The main two things done in this commit are:

 - Split large test methods which do several independent tests in
   blank code blocks into more smaller methods.

 - Fix an unnecessarily complicated pattern where the code would
   create a `Function` in a local variable and then immediately
   `apply` it exactly once... rather than just executing the code
   normally.

Author: PeteGillinElastic