deflake TestRestartMonitor

Fix #5146

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

restart_monitor_linux_test.go

@@ -82,16 +82,20 @@ version = 2
 	deadline := begin.Add(interval).Add(epsilon)
 	for time.Now().Before(deadline) {
 		status, err := task.Status(ctx)
+		now := time.Now()
 		if err != nil {
-			t.Fatal(err)
-		}
-		t.Logf("%v: status=%q", time.Now(), status)
+			// ErrNotFound is expected here, because the restart monitor
+			// temporarily removes the task before restarting.
+			t.Logf("%v: err=%v", now, err)
+		} else {
+			t.Logf("%v: status=%q", now, status)
 
 			if status.Status == Running {
 				elapsed := time.Since(begin)
-			t.Logf("the task was restarted after %s", elapsed.String())
+				t.Logf("the task was restarted within %s", elapsed.String())
 				return
 			}
+		}
 		time.Sleep(epsilon)
 	}
 	t.Fatalf("the task was not restarted in %s + %s",