Update test to cope with asynchronous writing of the access log

Previously, the test that verified that Undertow's access log could be
enabled and was written to the correct file expected the log to be
available as soon as the request to the server had completed. Undertow
writes the access log asynchronously so the test failed intermittently
due to a race between the access log being written and the test
asserting that it exists.

This commit updates the test to wait for up to 10 seconds for the
access log to be available.

See gh-4670

spring-boot/src/test/java/org/springframework/boot/context/embedded/undertow/UndertowEmbeddedServletContainerFactoryTests.java

@@ -178,7 +178,8 @@ public class UndertowEmbeddedServletContainerFactoryTests
 	}
 
 	@Test
-	public void accessLogCanBeEnabled() throws IOException, URISyntaxException {
+	public void accessLogCanBeEnabled()
+			throws IOException, URISyntaxException, InterruptedException {
 		UndertowEmbeddedServletContainerFactory factory = getFactory();
 		factory.setAccessLogEnabled(true);
 		File accessLogDirectory = this.temporaryFolder.getRoot();
@@ -188,8 +189,9 @@ public class UndertowEmbeddedServletContainerFactoryTests
 				new ServletRegistrationBean(new ExampleServlet(), "/hello"));
 		this.container.start();
 		assertThat(getResponse(getLocalUrl("/hello")), equalTo("Hello World"));
-		assertThat(accessLogDirectory.listFiles(),
-				is(arrayContaining(new File(accessLogDirectory, "access_log.log"))));
+		File accessLog = new File(accessLogDirectory, "access_log.log");
+		awaitFile(accessLog);
+		assertThat(accessLogDirectory.listFiles(), is(arrayContaining(accessLog)));
 	}
 
 	@Override
@@ -197,6 +199,13 @@ public class UndertowEmbeddedServletContainerFactoryTests
 		return null; // Undertow does not support JSPs
 	}
 
+	private void awaitFile(File file) throws InterruptedException {
+		long end = System.currentTimeMillis() + 10000;
+		while (!file.exists() && System.currentTimeMillis() < end) {
+			Thread.sleep(100);
+		}
+	}
+
 	private ServletContainer getServletContainerFromNewFactory() {
 		UndertowEmbeddedServletContainer undertow1 = (UndertowEmbeddedServletContainer) getFactory()
 				.getEmbeddedServletContainer();