Fix flaky nested virtual threads test by waiting for child spans

The test was using a CountDownLatch that only ensured child threads had
started executing, not that their @trace spans had completed. Replace
the latch with blockUntilChildSpansFinished(1) at each nesting level,
matching the pattern used by other tests in this class.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bm1549

dd-java-agent/instrumentation/java/java-lang/java-lang-21.0/src/test/java/testdog/trace/instrumentation/java/lang/jdk21/VirtualThreadApiInstrumentationTest.java

@@ -6,7 +6,6 @@
 
 import datadog.trace.agent.test.AbstractInstrumentationTest;
 import datadog.trace.api.Trace;
-import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ThreadFactory;
 import java.util.concurrent.TimeoutException;
 import org.junit.jupiter.api.DisplayName;
@@ -96,7 +95,6 @@ public void run() {
   @Test
   void testNestedVirtualThreads() throws InterruptedException, TimeoutException {
     Thread.Builder.OfVirtual threadBuilder = Thread.ofVirtual();
-    CountDownLatch latch = new CountDownLatch(3);
 
     new Runnable() {
       @Trace(operationName = "parent")
@@ -118,20 +116,18 @@ public void run() {
                               @Override
                               public void run() {
                                 System.out.println("complete");
-                                latch.countDown();
                               }
                             });
-                        latch.countDown();
+                        blockUntilChildSpansFinished(1);
                       }
                     });
-                latch.countDown();
+                blockUntilChildSpansFinished(1);
               }
             });
+        blockUntilChildSpansFinished(1);
       }
     }.run();
 
-    latch.await();
-
     assertTraces(
         trace(
             SORT_BY_START_TIME,