ProcedureTestingUtility.restart is broken sometimes after HBASE-28199

In ProcedureTestingUtility.restart, we will reuse the same ProcedureExecutor, so when restarting, we need to make sure that all procedures are not executed and then clear the scheduler.

But after HBASE-28199, we may add procedures back to scheduler after a CompletableFuture is completed, so even if all the PEWorker are terminated we could still add things to scheduler, which may break some tests.

We need to find a way to deal with this.