[wpinet] Add mDNS discovery tests and fix mDNS JNI bugs (#8682)

In https://github.com/wpilibsuite/allwpilib/issues/8681 we discovered
that multicast service types need to be valid (end with _tcp or _udp),
or else errors are silently swallowed. Let's make our C++ unit test use
a valid name and also check that it works. I think if we
should/shouldn't do this is up for debate still.

I also discovered two bugs in the JNI code that lead to incorrect
results being returned
- Return array index was always 0
- Use of JLocal for the return value seems to mean that the array will
always be NULL in java

wpinet/src/test/java/org/wpilib/net/MulticastServiceAnnouncerTest.java

@@ -0,0 +1,53 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+package org.wpilib.net;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.net.UnknownHostException;
+import java.util.ArrayList;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class MulticastServiceAnnouncerTest {
+  @Test
+  void emptyText() throws InterruptedException, UnknownHostException {
+    final String serviceName = "Foaksdfjasklfj";
+    final String serviceType = "_wpinotxt._tcp";
+    final int port = 12345;
+
+    try (MulticastServiceAnnouncer announcer =
+            new MulticastServiceAnnouncer(serviceName, serviceType, port);
+        MulticastServiceResolver resolver = new MulticastServiceResolver(serviceType)) {
+      assertTrue(announcer.hasImplementation() && resolver.hasImplementation());
+
+      announcer.start();
+      resolver.start();
+
+      List<ServiceData> allData = new ArrayList<>();
+
+      for (int i = 0; i < 10; i++) {
+        ServiceData[] data = resolver.getData();
+        if (data == null) {
+          continue;
+        }
+
+        allData.addAll(List.of(data));
+
+        if (!allData.isEmpty()) {
+          break;
+        }
+
+        Thread.sleep(1000);
+      }
+
+      assertFalse(allData.isEmpty(), "Expected at least one service entry, but got none");
+
+      resolver.stop();
+      announcer.stop();
+    }
+  }
+}