[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/main/native/cpp/jni/WPINetJNI.cpp

@@ -330,9 +330,10 @@ Java_org_wpilib_net_WPINetJNI_getMulticastServiceResolverData
     return serviceDataEmptyArray;
   }
 
-  JLocal<jobjectArray> returnData{
-      env, env->NewObjectArray(allData.size(), serviceDataCls, nullptr)};
+  jobjectArray returnData =
+      env->NewObjectArray(allData.size(), serviceDataCls, nullptr);
 
+  size_t index = 0;
   for (auto&& data : allData) {
     JLocal<jstring> serviceName{env, MakeJString(env, data.serviceName)};
     JLocal<jstring> hostName{env, MakeJString(env, data.hostName)};
@@ -340,7 +341,6 @@ Java_org_wpilib_net_WPINetJNI_getMulticastServiceResolverData
     wpi::util::SmallVector<std::string_view, 8> keysRef;
     wpi::util::SmallVector<std::string_view, 8> valuesRef;
 
-    size_t index = 0;
     for (auto&& txt : data.txt) {
       keysRef.emplace_back(txt.first);
       valuesRef.emplace_back(txt.second);