Run vtkm_cont unit tests on all devices
If you look at the unit testing code for the vtkm::cont package, you will see that there are several Testing*.h files that define the test in a device-generic way and for each implemented device there is a test source that includes this header file to run the test on that device. This is a bit of a hassle. Instead, we should move to the new dynamic device selection where we compile tests for all the devices at once and then select the device on the command line.