Investigate sporadic bursts in CPU and memory usage during batch cancel test

Running the `tests.test_solver.Submission.test_cancel_batch` test, sometimes causes CPU and/or memory usage to increase to 100%/3-4GiB during prolonged test time of few minutes. Usually the test runs within 1m, but it has been observed to take 10m.

It is possible the reason for this is SAPI/QPU load, i.e. user throttling. Even if the reason is SAPI user throttling, the CPU/memory impact should be curbed.