$this->drupalPostForm('admin/config/system/site-information',['site_slogan'=>'Test site slogan'],'Save configuration');
$this->drupalPostForm('admin/config/system/site-information/translate/af/edit',['translation[config_names][system.site][slogan]'=>'Test site slogan in Afrikaans'],'Save translation');
// Get the front page and ensure that the translated configuration appears.
$this->drupalGet('af');
$this->assertText('Test site slogan in Afrikaans');