[CI] Add testing against real services

[Slamdunk]

This library has never been tested in CI over external services for Storage protocols, nor for SMTP one.

ddeboer/imap for example sets up a Dovecot instance to test IMAP calls.

Dovecot would be useful since it supports both IMAP and POP3, and many authentication mechanism, including for example the XOAUTH2 one (https://doc.dovecot.org/configuration_manual/authentication/oauth2/) that is under development (see #197, #200), and that could be mocked instantiating a temporary OAuth2 server like https://oauth2.thephpleague.com/

Mail system is a beast, and while I know that setting up a real mail server in CI may introduce unwanted bugs and maintenance burdens, right now this library is basically untested because we lack a proper test infrastructure.