Add DEVICE_SOURCES to vtkm_unit_tests

The vtkm_unit_tests function in the CMake build now allows you to specify which files need to be compiled with a device compiler using the DEVICE_SOURCES argument. Previously, the only way to specify that unit tests needed to be compiled with a device compiler was to use the ALL_BACKENDS argument, which would automatically compile everything with the device compiler as well as test the code on all backends. ALL_BACKENDS is still supported, but it no longer changes the sources to be compiled with the device compiler.

Fixes #705 (closed)