Write tests for MySQL user-deletion

[rndmh3ro]

Is your feature request related to a problem? Please describe.
We need to write some tests for the MySQL user-deletion tasks since we already got two bugs.

Describe the solution you'd like
They should be written with Ansible asserts and tested in the verify-step of molecule.
Tests (not complete):

• user with password and without authentication string
• user with password and authentication string
• user with authentication string and without password
• user with authentication string and password
• user without authentication string and without password

Should be tested on different MySQL and MariaDB-versions (this should hopefully happen automatically with all tested operating systems:

• MySQL < 5.7.6
• MySQL > 5.7.6
• MariaDB < 10.4
• MariaDB > 10.4

Additional context
#444
#443

[schurzi]

The Testcases are mostly implemented in the linked PR. But two of them are currently impossible to create:

• user with password and without authentication string
• user with authentication string and without password

This is because in current MariaDB versions the mysql.user table is a view linked to the mysql.global_privs table. So it is not possible to generate testing data with these versions. Also the view definition leaves no options to create these cases in any other way I can think of. The view in my test database is defined as:

select 
...
    if(
        json_value(`global_priv`.`Priv`,'$.plugin') in ('mysql_native_password','mysql_old_password'), 
        ifnull(json_value(`global_priv`.`Priv`,'$.authentication_string'),
            ''
        ),
        ''
    ) AS `Password`,
    ifnull(json_value(`global_priv`.`Priv`,'$.authentication_string'),'') AS `authentication_string`,
...
from `global_priv`