CakePHP: write test for table on shell

I'm writing a plugin for CakePHP that import/export databases. This plugin has a shell and its index() method lists the databases already exported:

public function index(){    //Gets alla files    $files = BackupManager::index();    $this->out(__d('mysql_backup', 'Backup files found: {0}', count($files)));    if (!empty($files)) {        //Parses files        $files = array_map(function ($file) {            if (isset($file->compression) && !$file->compression) {                $file->compression = __d('mysql_backup', 'none');            }            $file->size = Number::toReadableSize($file->size);            return array_values((array)$file);        }, $files);        //Table headers        $headers = [            __d('mysql_backup', 'Filename'),            __d('mysql_backup', 'Extension'),            __d('mysql_backup', 'Compression'),            __d('mysql_backup', 'Size'),            __d('mysql_backup', 'Datetime'),        ];        $this->helper('table')->output(array_merge([$headers], $files));    }}

Output with no database:

$ bin/cake backupWelcome to CakePHP v3.3.8 Console---------------------------------------------------------------App : srcPath: /home/mirko/Server/mirkopagliai/src/PHP : 7.0.12-1---------------------------------------------------------------Backup files found: 0

Output with some databases:

$ bin/cake backupWelcome to CakePHP v3.3.8 Console---------------------------------------------------------------App : srcPath: /home/mirko/Server/mirkopagliai/src/PHP : 7.0.12-1---------------------------------------------------------------Backup files found: 2+-------------------------------------------+-----------+-------------+-----------+-----------------+| Filename                                  | Extension | Compression | Size      | Datetime        |+-------------------------------------------+-----------+-------------+-----------+-----------------+| backup_mirkopagliai_20161113110419.sql.gz | sql.gz    | gzip        | 51,05 KB  | 13/11/16, 11:04 || backup_mirkopagliai_20161113110414.sql    | sql       | none        | 150,93 KB | 13/11/16, 11:04 |+-------------------------------------------+-----------+-------------+-----------+-----------------+

Now I need to write some tests.
I wrote the test for no database code:

public function testIndexNoBackups(){    $this->io->expects($this->once())        ->method('out')        ->with('Backup files found: 0', 1);    $this->BackupShell->index();}

My problem is to write a test of whether the table output (second example).

For now I only wrote this:

public function testIndex(){    //Creates a database    (new BackupExport())->export();    $this->io->expects($this->once())        ->method('out')        ->with('Backup files found: 1', 1);    $this->BackupShell->index();}

Output:

$ phpunit tests/TestCase/Shell/BackupShellTest.php --filter testIndexPHPUnit 5.4.6 by Sebastian Bergmann and contributors.E.                                                                  2 / 2 (100%)Time: 114 ms, Memory: 6.00MBThere was 1 error:1) MysqlBackup\Test\TestCase\Shell\BackupShellTest::testIndexError: Call to a member function output() on null/home/mirko/Libs/Plugins/cakephp-mysql-backup/src/Shell/BackupShell.php:110/home/mirko/Libs/Plugins/cakephp-mysql-backup/tests/TestCase/Shell/BackupShellTest.php:191ERRORS!Tests: 2, Assertions: 1, Errors: 1.

So, how to write a test to check the table output? Where can I find an example?