Improve CLI commands tests coverage

[burhandodhy]

Description of the Change

This PR adds the new tests for the WP CLI commands. With this change, the coverage of commands is reached to 90%. 

I made one change in the put_mapping_helper instead of returning false when the error happens. I update that with WP_CLI::error so it stops the execution as soon as an error occurs. 

Also, this particular line

ElasticPress/includes/classes/Command.php

Line 1338 in 5d2dd65

WP_CLI::error( esc_html__( 'Mapping Failed.', 'elasticpress' ) );

fixes this issue #3198.

Closes #3198 #3054

How to test the Change

Changelog Entry

Added - New test for WP CLI Commands
Fixed - Sync command exits without any error message if mapping fails

Credits

Props @burhandodhy, @felipeelia

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.