Issue #3466743 by DYdave: Fixed fatal error 'Call to undefined method...
requested to merge issue/admin_toolbar-3466743:3466743-admintoolbartoolssettingsform-fix-fatal into 3.x
https://www.drupal.org/project/admin_toolbar/issues/3466743
Fixed fatal error Call to undefined method Drupal\Core\Menu\MenuLinkManager::invalidateAll() and added initial automated tests for the AdminToolbarToolsSettingsForm.