The problem is that currently there is no way to test the behavior
of the mysql_change_user() function using the mysqltest suite because
there is no internal command for it.

The solution is to introduce a change_user command that can be used
to test aspects of the MySQL client function mysql_change_user().


client/mysqltest.c:
  Add change_user command to mysqltest.
mysql-test/r/mysqltest.result:
  Add test case result for change_user command
mysql-test/t/mysqltest.test:
  Add test case for change_user command
mysql-test/r/change_user.result:
  Add new file with test case results for bugs 20023 and 31418.
mysql-test/t/change_user.test:
  Add new file with test cases for bugs 20023 and 31418.