More tests for CI/CD

[Diman0]

We have seen in the last couple of weeks multiple bugs that could have been prevented with better test coverage.

Let's gather in this issue the functionality that should have (better) tests.

• Config-export
• Config-import
• Webdav connectivity
• webmail
• pop3

config-export/import

For Config-export/import we could just do an export and then a replacing import.

webdav

For testing webdav, we could initialize the webdav container with an existing database for a user.
Then we use curl (?) to submit a webdav request to the webdav endpoint.
If the response contains the expected response code, then the test succeeds

webmail

We need two tests for both roundcube/snappymail.
enable header authentication
curl to submit a request for /webmail that is automatically logged in.
I think the result of the above curl should be the main page of roundcube/snappymail?