clEnqueueSVMMemFill is not properly tested

[karolherbst]

I just noticed today, that when the implementation of clEnqueueSVMMemFill is doing nothing it can make the CTS pass. I suspect this is usually only the case when allocated memory is zeroed by the OS (host memory or GPUs with unified memory). The tests using clEnqueueSVMMemFill seem to only compare between two allocations, but not to what's expected from the tests perspective. "svm_enqueue_api" is one of the tests impacted by this, "svm_migrate" might be as well.