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.
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