feat: Add time.Sleep to mitigate race condition.

[marianogappa]

The ShuffleQueue scheduler strategy has an infrequent race condition, as explained by the comment:

	// A race condition is possible when the last active table asynchronously
	// queues a relation. The table finishes (calling `.Done()`) a moment
	// before the queue receives the `.Push()`. At this point, the queue is
	// empty and there are no active workers.
	//
	// A moment later, the queue receives the `.Push()` and queues a new task.
	//
	// This is a very infrequent case according to tests, but it happens.

After many attempts at a more elegant solution, I finally yielded:

time.Sleep(10 * time.Millisecond)

Looks ugly, but after running the tests 300 times (so around 3000 syncs), it works 🤷

✓ cloudquery/plugin-sdk main* $ go test ./scheduler -count=100 -run TestScheduler             ⏱ 15:04:12
ok  	github.com/cloudquery/plugin-sdk/v4/scheduler	143.523s
✓ cloudquery/plugin-sdk main* $ go test ./scheduler -count=100 -run TestScheduler              ⏱ 15:06:56
ok  	github.com/cloudquery/plugin-sdk/v4/scheduler	142.796s
✓ cloudquery/plugin-sdk main* $ go test ./scheduler -count=100 -run TestScheduler              ⏱ 15:09:22
ok  	github.com/cloudquery/plugin-sdk/v4/scheduler	144.304s

[marianogappa]

lol, there's a unit test failure but that one wasn't this code:

panic: test timed out after 10m0s
	running tests:
		TestScheduler_Cancellation (9m51s)
		TestScheduler_Cancellation/should_not_consume_all_message_on_cancel_shuffle (9m51s)

Note the strategy is shuffle, not shuffle-queue:

t.Run(fmt.Sprintf("%s_%s", tc.name, strategy.String())