Allow more time for shutdown result callback to be received

5 seconds isn't always sufficient, particularly on CI where the timing
can be quite variable.

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/reactive/server/AbstractReactiveWebServerFactoryTests.java

@@ -380,7 +380,7 @@ public abstract class AbstractReactiveWebServerFactoryTests {
 		assertThat(responseReference.get()).isNull();
 		blockingHandler.completeOne();
 		assertThat(responseLatch.await(5, TimeUnit.SECONDS)).isTrue();
-		Awaitility.await().atMost(Duration.ofSeconds(5)).until(() -> GracefulShutdownResult.IDLE == result.get());
+		Awaitility.await().atMost(Duration.ofSeconds(30)).until(() -> GracefulShutdownResult.IDLE == result.get());
 	}
 
 	@Test