[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/cmake.yml

@@ -37,6 +37,12 @@ jobs:
         if: runner.os == 'Linux'
         run: sudo apt-get update && sudo apt-get install -y libopencv-dev libopencv-java ninja-build
 
+      - name: Setup avahi-daemon
+        if: runner.os == 'Linux'
+        run: |
+          sudo service dbus start
+          sudo avahi-daemon -D
+
       - name: Install dependencies (macOS)
         if: runner.os == 'macOS'
         run: brew install opencv ninja