Dot product test fails with missing event message

[Bensuo]

Summary of the issue:

The dot product test in the sycl-graph-poc-v2 branch currently fails to run correctly, with the following output:

die: The host-visible proxy event missing
terminate called without an active exception

Summary of findings:

• Issue is caused by the use of the reduction object in the last graph node. This creates a requirement for an auxiliary resource when the command is scheduled.
• When the aux resources are being cleaned up they wait for any command submissions to have finished, however the lazy queue command list is not correctly indicating that it has finished executing and at this point the runtime panics.
• It's unclear whether the command list is notifying correctly and this is just not being reflected in the host visible event associated with the command or if it is failing to notify altogether.
• Making sure that the graph command list correctly notifies the associated event when it is finished should fix the issue, though exactly where that change needs to happen is unclear to me.
• Existing queue mechanisms for resetting and cleaning up command lists may help to avoid code duplication, if they can be used with the separate lazy command list.

[EwanC]

PR add a test with minimal reproducer - #31

[Bensuo]

#46 adds a buffer based dotp example which fails with this same error when attempting to copy data back to host, suggesting this issue affects all uses of buffers and not just those used for auxilliary resources.