Fix wrong callback in `logger.go`, and add test cases:
1. `logger.Warningln` should call `logger.Warnln`, not `logger.Warn`.
2. It's ok for `logger.Print` to call `entry.Info`, but calling `entry.Print`
is better.
Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>