Cleanup CLI script and tests

[stevegrunwell]

This PR focus on cleaning up some of the less-obvious behaviors within the WP-CLI command and its corresponding tests:

1. Use @ticket instead of @link DocBlock annotations
2. Don't let the WP-CLI command suppress MySQL errors
3. Add a new debug message (e.g. requires --debug during execution) for the beginning of each batch
4. Explicitly reset the WP_CLI mock's static properties after each test
5. Rather than referencing the static properties of WP_CLI directly, add custom assertions to our dummy WP_CLI_Command, enabling things like $this->cli->assertReceivedMessage($msg[, $level])