servertest: add NNTP support

Summary:
Add the defines for the NNTP/NNTPS strings and ports;, implement the
basic conversation to get the capabilities of the server (if supported),
to know whether TLS is supported, and which authentication methods are
available.

Test Plan:
Run the servertest with some NNTP server, e.g. news.gmane.org,
nntp.perl.org, news.mozilla.org.

Reviewers: KDE PIM, mlaurent

Reviewed By: KDE PIM, mlaurent

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D6977