test: emacs: new tests "notmuch-show: {add,remove} multiple tags {to,from} single...

test: emacs: new tests "notmuch-show: {add,remove} multiple tags {to,from} single message"

* test/emacs:

  - Rename subtests "{Add,Remove} tag from notmuch-show view" to
    "notmuch-show: {add,remove} single tag {to,from} single message"
    to be consistent with the following tests.

  - New subtest "notmuch-show: add multiple tags to single message":
    `notmuch-show-add-tag' ("+") can add multiple tags to a message.

  - New subtest "notmuch-show: remove multiple tags from single message":
    `notmuch-show-remove-tag' ("-") can remove multiple tags from a message.