Wait for span to finish instead of ignoring test

dd-java-agent/instrumentation/apache-httpasyncclient-4/src/test/groovy/ApacheHttpAsyncClientNullCallbackTest.groovy

@@ -1,21 +1,14 @@
 import datadog.trace.agent.test.base.HttpClientTest
 import datadog.trace.instrumentation.apachehttpasyncclient.ApacheHttpAsyncClientDecorator
+import io.opentracing.util.GlobalTracer
 import org.apache.http.client.methods.HttpGet
 import org.apache.http.impl.nio.client.HttpAsyncClients
 import org.apache.http.message.BasicHeader
-import org.junit.Ignore
 import spock.lang.AutoCleanup
 import spock.lang.Shared
 
 import java.util.concurrent.Future
 
-/**
- * TODO: we would like not to ugnore this test, but currently this test is flaky
- * The problem is that http-request span is closed asynchronously and when we provide no callback (like 39)
- * we cannot synchronise on when it is closed. Possible soltion here would be to rewrite this to not run
- * tests will callbacks somehow because they make no sense in 'fire-and-forget' scenarios.
- */
-@Ignore
 class ApacheHttpAsyncClientNullCallbackTest extends HttpClientTest<ApacheHttpAsyncClientDecorator> {
 
   @AutoCleanup
@@ -41,6 +34,10 @@ class ApacheHttpAsyncClientNullCallbackTest extends HttpClientTest<ApacheHttpAsy
     Future future = client.execute(request, null)
     future.get()
     if (callback != null) {
+      // Request span is closed asynchronously even in regards to returned future so we have to wait here.
+      if (GlobalTracer.get().activeSpan() != null) {
+        blockUntilChildSpansFinished(1)
+      }
       callback()
     }
     return 200