Add vtkm_cont_testing to interface of vtkm_cont
Recent changes to VTK-m include changes to MakeDataSet.h that require the vtkm_cont_testing library to be loaded. This is problematic for software using an earlier minor revision as they do not expect to need to include this library. For now, include vtkm_cont_testing in the interface of vtkm_cont so that downstream code will load it. This should be a temporary solution. As we move to new versions of VTK-m, we should break this link and require downstream components to include it themselves.