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/mock_syscall.hpp b/test/mock_syscall.hpp
index d219999..f14bd96 100644
--- a/test/mock_syscall.hpp
+++ b/test/mock_syscall.hpp
@@ -1,4 +1,5 @@
 #pragma once
+#include <string>
 
 /** @brief Adds the given interface and addr info
  *         into the ifaddr list.
@@ -10,3 +11,10 @@
 
 void mock_addIP(const char* name, const char* addr, const char* mask,
                 unsigned int flags);
+
+/** @brief Adds an address string to index mapping
+ *
+ *  @param[in] name - Interface name
+ *  @param[in] idx  - Interface index
+ */
+void mock_addIF(const std::string& name, int idx);