Rename to avoid word "test"; toward a stateless SourceService

Our style guide (see Wiki) recommends not using "Test" as to refer to assessment or paper. But we have a bunch of "TestFoo" classes. This change focuses on TestSourceService.

This change also shows a workable model for gradually replacing the stateful code like this:

  tss = TestSourceService()
  ...
  tss.some_method()

with code:

  SourceService.some_method(...)

This was first introduced with SpecificationService and I've been wanting to move code to this model, but its daunting.

Here I've kept class TestSourceService around, changed its __init__ method to print Deprecated. Then I can slowly port the code, one or two methods at a time. Of course the new service can call the methods of the deprecated one and vice-versa during the transition.

(Probably I will finish this work in another MR so I can refer to this one later).