[3.13] gh-70039: smtplib: store the server name in ._host in .connect() (GH-115259) (#148272)

Original patch by gigaplastik, extended with a few more tests.

Addresses gh-70039 and bpo-25852: failure of starttls if connect is called explicitly.
(cherry picked from commit 442f83a5ea1b4d334befd231a79c40d6ff41a0bd)

Co-authored-by: nmartensen <nis.martensen@web.de>