test: fix UDPStreamer multicast tests broken by mandatory Interface

Commit 3e0a481 made Interface mandatory for multicast in both
UDPStreamer::Initialise and UDPSServer::Initialise and updated
Docs/UDPStreamer.md, but left the GTest configs untouched. All four
multicast tests have failed since.

Add Interface to the five multicast configs. The field is parsed with
inet_addr(), so it takes a dotted-quad, not an interface name; 127.0.0.1
keeps the tests self-contained and off the LAN.

TestInitialise_MulticastMode_InvalidDataPort was passing for the wrong
reason: UDPStreamer.cpp rejected it on the missing Interface before the
DataPort == Port check could run. It now proves what its name claims.

TestExecute_MulticastConnectDataDisconnect additionally needed the
two-argument Join(): the single-argument form passes INADDR_ANY, so the
reader joined the default-route interface while the server sent on
loopback via IP_MULTICAST_IF, and the DATA datagram never arrived.

GTest: 132/132 (was 128/132), multicast subset stable over 3 runs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Test/Components/DataSources/UDPStreamer/UDPStreamerTest.cpp

@@ -1559,6 +1559,7 @@ bool UDPStreamerTest::TestInitialise_MulticastMode_Valid() {
     ConfigurationDatabase cdb;
     cdb.Write("Port", 44710u);
     cdb.Write("MulticastGroup", "239.0.0.1");
+    cdb.Write("Interface", "127.0.0.1");
     cdb.Write("DataPort", 44711u);
     cdb.CreateRelative("Signals");
     cdb.MoveToRoot();
@@ -1574,6 +1575,7 @@ bool UDPStreamerTest::TestInitialise_MulticastMode_DefaultDataPort() {
     ConfigurationDatabase cdb;
     cdb.Write("Port", 44712u);
     cdb.Write("MulticastGroup", "239.0.0.1");
+    cdb.Write("Interface", "127.0.0.1");
     /* DataPort intentionally omitted: should default to 44713 */
     cdb.CreateRelative("Signals");
     cdb.MoveToRoot();
@@ -1588,6 +1590,9 @@ bool UDPStreamerTest::TestInitialise_MulticastMode_InvalidDataPort() {
     ConfigurationDatabase cdb;
     cdb.Write("Port", 44714u);
     cdb.Write("MulticastGroup", "239.0.0.1");
+    /* Interface is mandatory for multicast; supply it so the rejection below
+     * is provably caused by DataPort == Port and not by a missing Interface. */
+    cdb.Write("Interface", "127.0.0.1");
     cdb.Write("DataPort", 44714u); /* same as Port — must be rejected */
     cdb.CreateRelative("Signals");
     cdb.MoveToRoot();
@@ -1618,6 +1623,7 @@ bool UDPStreamerTest::TestPrepareNextState_Multicast() {
         "            Class = UDPStreamer\n"
         "            Port = 44716\n"
         "            MulticastGroup = \"239.0.0.1\"\n"
+        "            Interface = \"127.0.0.1\"\n"
         "            DataPort = 44717\n"
         "            MaxPayloadSize = 1400\n"
         "            Signals = {\n"
@@ -1695,6 +1701,7 @@ bool UDPStreamerTest::TestExecute_MulticastConnectDataDisconnect() {
         "            Class = UDPStreamer\n"
         "            Port = 44720\n"
         "            MulticastGroup = \"239.0.0.1\"\n"
+        "            Interface = \"127.0.0.1\"\n"
         "            DataPort = 44721\n"
         "            MaxPayloadSize = 1400\n"
         "            Signals = {\n"
@@ -1786,7 +1793,12 @@ bool UDPStreamerTest::TestExecute_MulticastConnectDataDisconnect() {
         ok = mcastReader.Listen(44721u);
     }
     if (ok) {
-        ok = mcastReader.Join("239.0.0.1");
+        /* Join on the same interface the streamer sends from (Interface =
+         * 127.0.0.1 sets IP_MULTICAST_IF on the server's data socket). The
+         * single-argument Join() would pass INADDR_ANY, letting the kernel
+         * pick the default-route interface, and the datagram would never
+         * reach this socket. */
+        ok = mcastReader.Join("239.0.0.1", "127.0.0.1");
     }
 
     /* Step 4: Trigger Synchronise() to generate a DATA packet */