Move MakeTestDataSet.cxx to vtkm_cont

Originally MakeTestDataSet.h was header-only code. This was changed to put its functionality into a library. The obvious library to put it in was vtkm_cont_testing.

The problem is that there is some external code that is (unadvisably) adding this header to their own code. This code ends up breaking because it needed to include vtkm_cont_testing, which was not needed before.

As a workaround, there was a CMake interface link that caused external code linking in vtkm_cont to also link in vtkm_cont_testing. This works OK, but creates something of a circular dependency. CMake itself has no problem with this, because vtkm_cont did not itself depend on vtkm_cont_testing; only things that linked to vtkm_cont also had to link to vtkm_cont_testing.

Unfortunately, some Ascent CMake script was trying to trace down dependencies and ran into an infinite loop. So this solution was not so great for them.

Get around the problem by just adding the MakeTestDataSet code directly to vtkm_cont (for now).