test: Show all headers in emacs reply tests

test: Show all headers in emacs reply tests

By default, emacs hides the User-Agent and References headers when
composing mail. This is a good thing for users, but a bad thing for
testing, since we can create ugly or invalid headers and not have it
show up in the tests.

By setting message-hidden-headers to an empty list, we force emacs to
show all the headers, so we can check that they're correct. Users
won't see this, but it will let us catch future bugs.

As a side-effect, this breaks all the reply tests, since there is a
bug with the References and User-Agent headers, fixed in the next commit.