[4206] Fixed Dhcp4 unit test failure under OS-X

src/bin/dhcp4/tests/dhcp4_srv_unittest.cc
    TEST_F(Dhcpv4SrvTest, emptyClientId)
        Added code to set up test interfaces

src/bin/dhcp4/tests/dhcp4_srv_unittest.cc

@@ -2663,6 +2663,8 @@ TEST_F(Dhcpv4SrvTest, statisticsUnknownRcvd) {
 // This test verifies that the server is able to handle an empty client-id
 // in incoming client message.
 TEST_F(Dhcpv4SrvTest, emptyClientId) {
+    IfaceMgrTestConfig test_config(true);
+    IfaceMgr::instance().openSockets4();
     Dhcp4Client client;
 
     EXPECT_NO_THROW(configure(CONFIGS[0], *client.getServer()));