Some of the code in the base `vtkm` namespace is device specific. For
example, the functions in `Math.h` are customized for specific devices.
Thus, we want this code to be specially compiled and run on these
devices.
Previously, we made a header file and then added separate tests to each
device package. That was created before we had ways of running on any
device. Now, it is much easier to compile the test a single time for all
devices and use the `ALL_BACKENDS` feature of `vtkm_unit_tests` CMake
function to automatically create the test for all devices.