[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/native/cpp/MulticastTest.cpp

@@ -3,11 +3,10 @@
 // the WPILib BSD license file in the root directory of this project.
 
 #include <array>
-#include <chrono>
 #include <string>
 #include <string_view>
 #include <thread>
-#include <utility>
+#include <vector>
 
 #include <gtest/gtest.h>
 
@@ -17,7 +16,7 @@
 
 TEST(MulticastServiceAnnouncerTest, EmptyText) {
   const std::string_view serviceName = "TestServiceNoText";
-  const std::string_view serviceType = "_wpinotxt";
+  const std::string_view serviceType = "_wpinotxt._tcp";
   const int port = std::rand();
   wpi::net::MulticastServiceAnnouncer announcer(serviceName, serviceType, port);
   wpi::net::MulticastServiceResolver resolver(serviceType);
@@ -26,7 +25,20 @@ TEST(MulticastServiceAnnouncerTest, EmptyText) {
     announcer.Start();
     resolver.Start();
 
-    std::this_thread::sleep_for(std::chrono::seconds(1));
+    std::vector<wpi::net::MulticastServiceResolver::ServiceData> allData;
+
+    for (int i = 0; i < 15; i++) {
+      // GetData gives me new data since last time. This smoketest is just
+      // looking for -any- response
+      allData = resolver.GetData();
+      if (!allData.empty()) {
+        break;
+      }
+
+      std::this_thread::sleep_for(std::chrono::seconds(1));
+    }
+
+    ASSERT_GT(allData.size(), 0ul);
 
     resolver.Stop();
     announcer.Stop();