face: deduplicate multicast UDP socket setup

In preparation for adding IPv6 support.

Change-Id: I5a3be2007f7fecc44915a5d6794093143ce9c0f9
Refs: #4222
diff --git a/tests/daemon/face/multicast-udp-transport.t.cpp b/tests/daemon/face/multicast-udp-transport.t.cpp
index 6e4a8cf..51acb08 100644
--- a/tests/daemon/face/multicast-udp-transport.t.cpp
+++ b/tests/daemon/face/multicast-udp-transport.t.cpp
@@ -79,10 +79,7 @@
 
   // remoteSockRx2 unnecessary for this test case - only remoteSockTx2 is needed
   udp::socket remoteSockTx2(this->g_io);
-  remoteSockTx2.open(udp::v4());
-  remoteSockTx2.set_option(udp::socket::reuse_address(true));
-  remoteSockTx2.set_option(ip::multicast::enable_loopback(true));
-  remoteSockTx2.bind(udp::endpoint(ip::address_v4::any(), 7071));
+  MulticastUdpTransport::openTxSocket(remoteSockTx2, udp::endpoint(udp::v4(), 7071), true);
 
   Block pkt1 = ndn::encoding::makeStringBlock(300, "hello");
   ndn::Buffer buf1(pkt1.begin(), pkt1.end());