* test/emacs:
- New subtest "notmuch-show: show message headers":
Setting `notmuch-message-headers-visible' to t causes all headers
defined in `notmuch-message-headers' to be shown.
- New subtest "notmuch-show: hide message headers":
Setting `notmuch-message-headers-visible' to nil causes all headers
defined in `notmuch-message-headers' to be hidden.
("Subject:" may be an exception; See the use of `headers-start' in
`notmuch-show-insert-msg')
- New subtest "notmuch-show: hide message headers (w/ notmuch-show-toggle-headers)":
Setting `notmuch-message-headers-visible' to t causes all headers
defined in `notmuch-message-headers' to be shown, but they can be
hidden for the current message by running `notmuch-show-toggle-headers'.
