[Symfony 6] Drupal\Core\Routing\LazyRouteCollection::all(): Return value must be of type array, ArrayIterator returned

Problem/Motivation

In #3161889: [META] Symfony 6 compatibility once Symfony 6 is installed, some tests fail with

1) Drupal\Tests\Core\Routing\LazyRouteCollectionTest::testGetIterator
TypeError: Drupal\Core\Routing\LazyRouteCollection::all(): Return value must be of type array, ArrayIterator returned

Symfony has added a return type of array to Symfony\Component\Routing\RouteCollection::all(), but our lazy route collection test wants to provide an iterator instead.

Steps to reproduce

Proposed resolution

Change the test to use an array instead of an iterator.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet