We setup the currently latest version of hMailServer[1] as mail server,
and configure according to imap_include.inc. To not require further
configuration, we adapt mail_skipif.inc and mail_include.inc to match
that configuration. However, we also change the default domain to the
reserved `example.com`.
We also update the standard mail tests to use the `--EXTENSIONS--`
section (instead of skipping the tests if ext/imap is not available).
Finally, we fix bug80751.phpt to expect the configured To and Cc mail
addresses.
[1] <https://www.hmailserver.com/>
Closes GH-8357.
