Cpu page-table updates and fixes

There were flaws in the engine idle computation used to determine whether we could use CPU page-table updates. Fix those

It also turns out fixing those triggers CPU-page-table updates in the migrate test, which didn't really support that, so also fix the migrate test to handle that and to run the test twice, once with CPU- and once with GPU page-table updates, and to also provide some timings for non-synced updates. (The timings avoid the GPU submission latency to better reflect the case where we are able to pipeline GPU page-table updates).

Finally provide some coverage for the fence seqno 32-bit wrapping case by setting the initial fence seqno to a small negative number.