testing: remove vtkm/cont/testing relative path

From the perspective of a C++ source file, the relative path of Testing.h is unknown, the only thing that we can assume is that it is located in the same directory as this header file. This is because the testing directory is reserved for test executables and not libraries, the vtkm_cont_testing module has to put this file in vtkm/cont/testlib instead of vtkm/cont/testing where you normally would expect it.

Backport: release