added missing assert to mm execution test in auto tuning

[johanneslenfers]

The mm example execution as part of the auto tuning tests is a heavy workload if running on a non-gpu device. Therefore, the execution is allowed to fail with an EXECUTION_ERROR. This PR adds the missing assert.

[Bastacyclop]

I think ideally tests should be able to discriminate between different execution errors. For example in this case I think the test should only be allowed to pass if the CL_OUT_OF_RESOURCES error is triggered?

The next step after that would be to query things like clGetDeviceInfo(device, CL_DEVICE_MAX_WORK_ITEM_DIMENSIONS, ...), CL_DEVICE_MAX_WORK_ITEM_SIZES, CL_DEVICE_MAX_WORK_GROUP_SIZES, CL_DEVICE_LOCAL_MEM_SIZE, ...
And either check them against clGetKernelWorkGroupInfo data, or even better allow this info to drive autotuning (as was done in Lift I believe, at least to some extent).