CMake: adds VTKm_ENABLE_TESTING_LIBRARY
In some projects such as VTK we have unit tests which needs
vtkm_cont_testing_library headers and exports.
This is only enabled when we set
VTKm_ENABLE_TESTING, however, often times, those projects do not need to build all of the VTKm unit tests.
For that reason we need one more CMake Variable to only enable
vtkm_cont_testing_library without building the tests. For that I propose the new option/variable