test: emacs: new tests "notmuch-show: {, un}collapse all messages in thread"

* test/emacs:

  - New subtest "notmuch-show: collapse all messages in thread":
    `notmuch-show-open-or-close-all' with prefix arg ("C-u M-RET")
    collapses all messages in thread.

  - New subtest "notmuch-show: uncollapse all messages in thread":
    `notmuch-show-open-or-close-all' without prefix arg ("M-RET")
    uncollapses all messages in thread.