Reduce flakiness of LayoutIntegrationTests (#3537)

Two of the tests were waiting for log output using a one-off call to `Thread.sleep()`.

In addition to being brittle, the tests would fail with an NPE (while trying to assert on the "timestamp" field) that doesn't point to the cause of the failure directly.

Also removed the redundant null-check as `objectMapper.readTree()` will never return `null` in Jackson 2.10.x, see FasterXML/jackson-databind#2211

(cherry picked from commit bdb829c)

dropwizard-json-logging/pom.xml

@@ -116,6 +116,11 @@
             <artifactId>dropwizard-configuration</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.awaitility</groupId>
+            <artifactId>awaitility</artifactId>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
 </project>

dropwizard-json-logging/src/test/java/io/dropwizard/logging/json/LayoutIntegrationTests.java

@@ -37,6 +37,7 @@
 import static java.util.Objects.requireNonNull;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.entry;
+import static org.awaitility.Awaitility.await;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
 
@@ -149,10 +150,10 @@ public void testLogJsonToConsole() throws Exception {
             defaultLoggingFactory.configure(new MetricRegistry(), "json-log-test");
             Marker marker = MarkerFactory.getMarker("marker");
             LoggerFactory.getLogger("com.example.app").info(marker, "Application log");
-            Thread.sleep(100); // Need to wait, because the logger is async
+            // Need to wait, because the logger is async
+            await().atMost(1, TimeUnit.SECONDS).until(() -> !redirectedStream.toString().isEmpty());
 
             JsonNode jsonNode = objectMapper.readTree(redirectedStream.toString());
-            assertThat(jsonNode).isNotNull();
             assertThat(jsonNode.get("timestamp").isTextual()).isTrue();
             assertThat(jsonNode.get("level").asText()).isEqualTo("INFO");
             assertThat(jsonNode.get("logger").asText()).isEqualTo("com.example.app");
@@ -204,10 +205,10 @@ public void testLogAccessJsonToConsole() throws Exception {
             when(response.getHeader("Server")).thenReturn("Apache/2.4.12");
 
             requestLog.log(request, response);
-            Thread.sleep(100); // Need to wait, because the logger is async
+            // Need to wait, because the logger is async
+            await().atMost(1, TimeUnit.SECONDS).until(() -> !redirectedStream.toString().isEmpty());
 
             JsonNode jsonNode = objectMapper.readTree(redirectedStream.toString());
-            assertThat(jsonNode).isNotNull();
             assertThat(jsonNode.get("timestamp").isNumber()).isTrue();
             assertThat(jsonNode.get("requestTime").isNumber()).isTrue();
             assertThat(jsonNode.get("remoteAddress").asText()).isEqualTo("10.0.0.1");