What is the privilege for account unlock in Oracle
you have not shown the roles TEST_USER is granted.  Likely they have been granted the DBA role which allows ALTER USER.

see the documentation for details.  This query will show the roles a user has

SELECT * FROM DBA_ROLE_PRIVS where grantee = 'TEST_USER'


of course this requires privileges too, unless you have the CATALOG role, if you select from this table you can alter a user