test: set mail host in emacs_deliver

test: set mail host in emacs_deliver

One test (reply to encrypted message in the crypto test) recently
started failing on some systems. The failure I saw were two extra
lines of the form
<87d2nbc5xg.fsf@host.i-did-not-set--mail-host-address--so-tickle-me>

The test pipes the output through

grep -v -e '^In-Reply-To:' -e '^References:'

which would normally these two ids but it does not, in this case,
because they are so long they get put on a separate line in the output.

To fix this we set mail-host-address for emacs deliver. example.com
seems a sensible address to use. This is short enough that we don't
get the line breaks above and the tests then all pass.