Add integration tests for creating and updating a mail share

In most cases, when a mail share is created or updated an e-mail is sent
to the sharee, which is done by connecting to the SMTP server set in the
configuration. If the server can not be contacted then the creation or
update of the mail share fails.

To make possible to test mail shares without using a real SMTP server a
fake one has been added. The original script, which is MIT licensed, was
based on inetd, so it was slightly modified to run on its own.

In order to use it from the integration tests the "Given dummy mail
server is listening" step has to be called in the scenarios in which the
mail server is needed.

For now that is the only available step; things like checking the sent
mails, while possible (as the script can log the mails to certain file),
have not been added yet.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>