renameUser: add unit test

Like the previous change, this test added unit test for |renameUser|. To
make it happen, this commit introduces a virtual function
|executeUserModify| which is overridden in unit test (see previous
change for why we need such overriding).

Tested: unit test passed.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I230a8c12fd6c86d0fccfc00872eff647473c1e94
diff --git a/user_mgr.hpp b/user_mgr.hpp
index 0195de8..b057bf0 100644
--- a/user_mgr.hpp
+++ b/user_mgr.hpp
@@ -313,6 +313,9 @@
 
     virtual void executeUserDelete(const char* userName);
 
+    virtual void executeUserRename(const char* userName,
+                                   const char* newUserName);
+
     /** @brief check for valid privielge
      *  method to check valid privilege, and throw if invalid
      *