Make 'steal' command atomic (#1144)

Either unschedule all requested tests, or none if it's not possible -
if some of the requested tests have already been processed by the time
the request arrives. It may happen if the worker runs tests faster than
the controller receives and processes status updates. But in this case
maybe it's just better to let the worker keep running.

This is a prerequisite for group/scope support in worksteal scheduler -
so they won't be broken up incorrectly.

This change could break schedulers that use "steal" command. However:

1) worksteal scheduler doesn't need any adjustments.

2) I'm not aware of any external schedulers relying on this command yet.

So I think it's better to keep the protocol simple, not complicate it for
imaginary compatibility with some unknown and likely non-existent
schedulers.

Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>

testing/test_remote.py

@@ -267,6 +267,12 @@ class TestWorkerInteractor:
 
         worker.sendcommand("steal", indices=[1, 2])
         ev = worker.popevent("unscheduled")
+        # Cannot steal index 1 because it is completed already, so do not steal any.
+        assert ev.kwargs["indices"] == []
+
+        # Index 2 can be stolen, as it is still pending.
+        worker.sendcommand("steal", indices=[2])
+        ev = worker.popevent("unscheduled")
         assert ev.kwargs["indices"] == [2]
 
         reports = [