Concurent instrumentation tests: block thread before starting tracing

Otherwise blocking thread is pretty much useless since we may still
get spans if threads are executed quickly enough.

dd-java-agent/instrumentation/java-concurrent/src/test/java/AsyncChild.java

@@ -31,12 +31,12 @@ public class AsyncChild implements Runnable, Callable {
   }
 
   private void runImpl() {
-    if (doTraceableWork) {
-      asyncChild();
-    }
     while (blockThread.get()) {
       // busy-wait to block thread
     }
+    if (doTraceableWork) {
+      asyncChild();
+    }
   }
 
   @Trace(operationName = "asyncChild")