clEnqueueSVMMemFill is not properly tested

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.

Start with the svm_enqueue_api test and inspect how clEnqueueSVMMemFill results are checked; review svm_migrate for the same gap. Compare each result with the expected fill value rather than only another allocation, and confirm the affected tests fail when the operation does nothing.

Written by the indexing model from the issue text.