name constraint check: add unit tests

This commit adds unit test for |ThrowForUserNameConstraints|. All lines
are covered by test now.

Tested: unit test passed

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: If1a0ff823ca0767f068030f205efdc1eb33e3029
diff --git a/user_mgr.hpp b/user_mgr.hpp
index dd73fff..d298bf0 100644
--- a/user_mgr.hpp
+++ b/user_mgr.hpp
@@ -246,6 +246,16 @@
      */
     void throwForUserExists(const std::string& userName);
 
+    /** @brief check user name constraints
+     *  method to check user name constraints and throw if failed.
+     *
+     *  @param[in] userName - name of the user
+     *  @param[in] groupNames - user groups
+     */
+    void
+        throwForUserNameConstraints(const std::string& userName,
+                                    const std::vector<std::string>& groupNames);
+
   private:
     /** @brief sdbusplus handler */
     sdbusplus::bus_t& bus;
@@ -281,16 +291,6 @@
      */
     UserSSHLists getUserAndSshGrpList(void);
 
-    /** @brief check user name constraints
-     *  method to check user name constraints and throw if failed.
-     *
-     *  @param[in] userName - name of the user
-     *  @param[in] groupNames - user groups
-     */
-    void
-        throwForUserNameConstraints(const std::string& userName,
-                                    const std::vector<std::string>& groupNames);
-
     /** @brief check group user count
      *  method to check max group user count, and throw if limit reached
      *