[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

.github/workflows/gradle.yml

@@ -60,7 +60,11 @@ jobs:
         with:
           image: ${{ matrix.container }}
           options: -v ${{ github.workspace }}:/work -w /work -e ARTIFACTORY_PUBLISH_USERNAME -e ARTIFACTORY_PUBLISH_PASSWORD -e GITHUB_REF -e CI
-          run: ./gradlew build --build-cache -PbuildServer -PskipJavaFormat ${{ matrix.build-options }} ${{ env.EXTRA_GRADLE_ARGS }}
+          # Start avahi-daemon and build
+          run: |
+            service dbus start
+            avahi-daemon -D
+            ./gradlew build --build-cache -PbuildServer -PskipJavaFormat ${{ matrix.build-options }} ${{ env.EXTRA_GRADLE_ARGS }}
         env:
           ARTIFACTORY_PUBLISH_USERNAME: ${{ secrets.ARTIFACTORY_USERNAME }}
           ARTIFACTORY_PUBLISH_PASSWORD: ${{ secrets.ARTIFACTORY_PASSWORD }}