Consolidate tests for base vtkm code that is device-specific

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.