Test Suite - Quota Tests

[Mischback]

Follow up to #1

Create Test Suite to verify the quota setup

The configuration applies a general quota to all mailboxes. User-specific quotas may be applied using dovecot's userdb (passwd-like file).

The server automatically notifies the user about quota limits (as of now at 80% and 95% of the quota).

The test suite should check for these notifications.

Implementation Idea

• [ ] use a dedicated user / mailbox for the test
• [ ] limit capacity to a known (and easily reachable) value (e.g. 1kB)
• [ ] manually reach a limit of 80% of the capacity
• [ ] manually reach 95% of the capacity
• [ ] add notification mails to expected_messages that will be checked during the (POP3-based) testing stage