eviction: Deflake TestStart

TestStart was previously flaky. In approx 100_000 local runs, it failed
about 70% of the time, and has been mentioned as a flaky unit test in
the past.

This flake was due to a race condition with the logic as written and the
go scheduler. UpdateThreshold calls `notifier.Start(events)` in a new Go
Routine, which is not guarunteed to be called immediately.

This meant that if `m.Start()` was called before `notifier.Start()`, the
test would fail, as the notifier would not have been started before the
4 events were processed and lock released.

Here, we update the test to more closely match the intended application
behaviour, and have events passed to the channel when `Start` is called
on the notifier.

This ensures that -Start gets called and additionally validates
that the correct channel is provided to the notifier.

Stop was never called previously, as it only gets called on a subsequent
call to UpdateThreshold. `AnyTimes()` hid that this did not occur.

pkg/kubelet/eviction/memory_threshold_notifier_test.go

@@ -174,8 +174,11 @@ func TestStart(t *testing.T) {
 	notifierFactory.EXPECT().NewCgroupNotifier(testCgroupPath, memoryUsageAttribute, int64(0)).Return(notifier, nil).Times(1)
 
 	var events chan<- struct{} = m.events
-	notifier.EXPECT().Start(events).Return()
-	notifier.EXPECT().Stop().Return().AnyTimes()
+	notifier.EXPECT().Start(events).DoAndReturn(func(events chan<- struct{}) {
+		for i := 0; i < 4; i++ {
+			events <- struct{}{}
+		}
+	})
 
 	err := m.UpdateThreshold(nodeSummary(noResources, noResources, noResources, isAllocatableEvictionThreshold(threshold)))
 	if err != nil {
@@ -184,9 +187,6 @@ func TestStart(t *testing.T) {
 
 	go m.Start()
 
-	for i := 0; i < 4; i++ {
-		m.events <- struct{}{}
-	}
 	wg.Wait()
 }