Add a way to capture mails sent through the mailer.transport service during tests

Pushed code removed in 📌 [PP-1] Add symfony mailer transports to DIC Postponed . I think this could be much less complex if we'd just replace the mailer.transport service directly with the capturing mail transport. Then the mailer_dsn config wouldn't have any effect at all and we wouldn't need to override the override it in tests.