Migrate OtlpWriterCombinedTest from groovy to junit

dd-trace-core/src/test/java/datadog/trace/common/writer/OtlpWriterCombinedTest.java

@@ -0,0 +1,56 @@
+package datadog.trace.common.writer;
+
+import static datadog.trace.api.config.OtlpConfig.TRACE_OTEL_EXPORTER;
+import static datadog.trace.junit.utils.config.WithConfigExtension.injectSysConfig;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import com.sun.net.httpserver.HttpServer;
+import datadog.trace.core.CoreTracer;
+import datadog.trace.core.DDCoreJavaSpecification;
+import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.Timeout;
+
+@Timeout(value = 10, unit = TimeUnit.SECONDS)
+class OtlpWriterCombinedTest extends DDCoreJavaSpecification {
+
+  @Test
+  void happyPathOverHttp() throws IOException, InterruptedException {
+    injectSysConfig(TRACE_OTEL_EXPORTER, "otlp");
+
+    CountDownLatch received = new CountDownLatch(1);
+    HttpServer server = HttpServer.create(new InetSocketAddress("localhost", 0), 0);
+    server.createContext(
+        "/v1/traces",
+        exchange -> {
+          received.countDown();
+          exchange.sendResponseHeaders(200, -1);
+          exchange.close();
+        });
+    server.start();
+
+    OtlpWriter writer =
+        OtlpWriter.builder()
+            .endpoint(
+                "http://"
+                    + server.getAddress().getHostString()
+                    + ":"
+                    + server.getAddress().getPort()
+                    + "/v1/traces")
+            .flushIntervalMilliseconds(-1)
+            .build();
+    CoreTracer tracer = tracerBuilder().writer(writer).build();
+    try {
+      tracer.buildSpan("test", "fakeOperation").start().finish();
+      writer.flush();
+
+      assertTrue(received.await(5, TimeUnit.SECONDS), "OTLP server should receive a request");
+    } finally {
+      tracer.close();
+      server.stop(0);
+    }
+  }
+}