fix unstable tests

Author: joshiste

spring-boot-admin-server/src/test/java/de/codecentric/boot/admin/server/config/AdminServerNotifierAutoConfigurationTest.java

@@ -63,17 +63,18 @@ public void close() {
  }
 
  @Test
- public void test_notifierListener() {
+ public void test_notifierListener() throws InterruptedException {
  load(TestSingleNotifierConfig.class);
  InstanceEventStore store = context.getBean(InstanceEventStore.class);
 
  StepVerifier.create(store)
  .expectSubscription()
  .then(() -> StepVerifier.create(store.append(Collections.singletonList(APP_DOWN))).verifyComplete())
  .expectNext(APP_DOWN)
- .then(() -> assertThat(context.getBean(TestNotifier.class).getEvents()).containsOnly(APP_DOWN))
  .thenCancel()
  .verify();
+ Thread.sleep(50); //wait for the notifications in different thread
+ assertThat(context.getBean(TestNotifier.class).getEvents()).containsOnly(APP_DOWN);
  }
 
  @Test
@@ -163,7 +164,6 @@ public static class TestSingleNotifierConfig {
  public Notifier testNotifier() {
  return new TestNotifier();
  }
-
  }
 
  private static class MailSenderConfig {