ThrowForInvalidPrivilege: add unit test

This commit adds unit test for the |throwForInvalidPrivilege| function.

Tested: unit test passed

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I0722ce2483adeffd493528c9c952aea9af181de6
diff --git a/user_mgr.hpp b/user_mgr.hpp
index c0ebe0b..9f2770d 100644
--- a/user_mgr.hpp
+++ b/user_mgr.hpp
@@ -313,6 +313,13 @@
 
     virtual void executeUserDelete(const char* userName);
 
+    /** @brief check for valid privielge
+     *  method to check valid privilege, and throw if invalid
+     *
+     *  @param[in] priv - privilege of the user
+     */
+    void throwForInvalidPrivilege(const std::string& priv);
+
   private:
     /** @brief sdbusplus handler */
     sdbusplus::bus_t& bus;
@@ -348,13 +355,6 @@
      */
     UserSSHLists getUserAndSshGrpList(void);
 
-    /** @brief check for valid privielge
-     *  method to check valid privilege, and throw if invalid
-     *
-     *  @param[in] priv - privilege of the user
-     */
-    void throwForInvalidPrivilege(const std::string& priv);
-
     /** @brief check for valid groups
      *  method to check valid groups, and throw if invalid
      *