Try to stabilize RestarterTests.testRestart

Closes gh-26900

spring-boot-project/spring-boot-devtools/src/test/java/org/springframework/boot/devtools/restart/RestarterTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,10 +18,12 @@ package org.springframework.boot.devtools.restart;
 
 import java.net.URL;
 import java.net.URLClassLoader;
+import java.time.Duration;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.concurrent.ThreadFactory;
 
+import org.awaitility.Awaitility;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -80,10 +82,11 @@ class RestarterTests {
 		Restarter.clearInstance();
 		Thread thread = new Thread(SampleApplication::main);
 		thread.start();
-		Thread.sleep(2600);
-		assertThat(StringUtils.countOccurrencesOf(output.toString(), "Tick 0")).isGreaterThan(1);
-		assertThat(StringUtils.countOccurrencesOf(output.toString(), "Tick 1")).isGreaterThan(1);
-		assertThat(CloseCountingApplicationListener.closed).isGreaterThan(0);
+		Awaitility.await().atMost(Duration.ofSeconds(30)).untilAsserted(() -> {
+			assertThat(StringUtils.countOccurrencesOf(output.toString(), "Tick 0")).isGreaterThan(1);
+			assertThat(StringUtils.countOccurrencesOf(output.toString(), "Tick 1")).isGreaterThan(1);
+			assertThat(CloseCountingApplicationListener.closed).isGreaterThan(0);
+		});
 	}
 
 	@Test