backend: Fix unreliable TestListCancel/Timeout test

The test uses `WithTimeout` to create a context that cancels the List
operation after a given delay. Several backends internally use a derived
child context created using WithCancel.

The cancellation of a context first closes the done channel of the
context (here: the `WithTimeout` context) and _afterwards_ propagates
the cancellation to child contexts (here: the `WithCancel` context).
Therefor if the List implementation uses a child context, then it may
take a moment until that context is also cancelled. Thus give the
context cancellation a moment to propagate.

internal/backend/test/tests.go

@@ -428,6 +428,11 @@ func (s *Suite[C]) TestListCancel(t *testing.T) {
 
 			// wait until the context is cancelled
 			<-ctxTimeout.Done()
+			// The cancellation of a context first closes the done channel of the context and
+			// _afterwards_ propagates the cancellation to child contexts. If the List
+			// implementation uses a child context, then it may take a moment until that context
+			// is also cancelled. Thus give the context cancellation a moment to propagate.
+			time.Sleep(time.Millisecond)
 			return nil
 		})