How to log into the database under the user? - psql

Question

I install postres9 on linux mint 18. I login under default postres user. I try create new user and login under this user.
kalinin#lenovo ~ $ sudo -i -u postgres
postgres#lenovo ~ $ psql
psql (9.5.16)
Type "help" for help.
postgres=# CREATE DATABASE db_test;
CREATE DATABASE
postgres=# CREATE USER user_test WITH PASSWORD 'qwerty';
CREATE ROLE
postgres=#
postgres=#
postgres=# GRANT ALL PRIVILEGES ON DATABASE "db_test" to user_test;
GRANT
postgres=# \du
List of roles
Role name | Attributes | Member of
-----------+------------------------------------------------------------+-----------
postgres | Superuser, Create role, Create DB, Replication, Bypass RLS | {}
user_test | | {}
postgres=# \l
List of databases
Name | Owner | Encoding | Collate | Ctype | Access privileges
-------------+----------+----------+-------------+-------------+------------------------
db_test | postgres | UTF8 | ru_RU.UTF-8 | ru_RU.UTF-8 | =Tc/postgres +
| | | | | postgres=CTc/postgres +
| | | | | user_test=CTc/postgres
postgres | postgres | UTF8 | ru_RU.UTF-8 | ru_RU.UTF-8 |
postgres=# \q
postgres#lenovo ~ $ exit
logout
kalinin#lenovo ~ $ sudo -i -u user_test
sudo: unknown user: user_test
sudo: unable to initialize policy plugin
you see what i am creating new user with password and new database. after that i exit from postgres and try login again. But login failed. Please help fix it.