userLockedForFailedAttempt: add unit test

This commit adds unit test for the |userLockedForFailedAttempt|
function. In order to do so, this commits adds a new virtual function
which reads the failed attempt. The function is overridden in unit test.

Tested: unit test passed.

Coverage:
  lines......: 83.1% (2007 of 2415 lines)
  functions..: 94.1% (430 of 457 functions)
  branches...: 32.3% (3181 of 9855 branches)

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I940ec42936c8b1c387fc0f19db13bc90a61c9852
diff --git a/user_mgr.hpp b/user_mgr.hpp
index 3f0d59c..6f7c7ce 100644
--- a/user_mgr.hpp
+++ b/user_mgr.hpp
@@ -323,6 +323,8 @@
     virtual void executeUserModifyUserEnable(const char* userName,
                                              bool enabled);
 
+    virtual std::vector<std::string> getFailedAttempt(const char* userName);
+
     /** @brief check for valid privielge
      *  method to check valid privilege, and throw if invalid
      *