test: Mock out interface lookups

This will make it possible to do interface lookups inside of our classes
during testing without depending on real interfaces existing.

Change-Id: I4c273d6961fa4229401fb25a0e5eb06af5b59ca4
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/test_ethernet_interface.cpp b/test/test_ethernet_interface.cpp
index 9e611a6..a075423 100644
--- a/test/test_ethernet_interface.cpp
+++ b/test/test_ethernet_interface.cpp
@@ -29,7 +29,7 @@
     TestEthernetInterface() :
         bus(sdbusplus::bus::new_default()),
         manager(bus, "/xyz/openbmc_test/network", "/tmp/"),
-        interface(bus, "/xyz/openbmc_test/network/test0", false, manager)
+        interface(makeInterface(bus, manager))
 
     {
         setConfDir();
@@ -50,6 +50,13 @@
         }
     }
 
+    static EthernetInterface makeInterface(sdbusplus::bus::bus& bus,
+                                           MockManager& manager)
+    {
+        mock_addIF("test0", 1);
+        return {bus, "/xyz/openbmc_test/network/test0", false, manager};
+    }
+
     int countIPObjects()
     {
         return interface.getAddresses().size();