test: add emacs test for hiding a message following an HTML part

test: add emacs test for hiding a message following an HTML part

Human-friendly scenario:

* open a thread where a message which ends with an HTML part is
  followed by another message

* make the first message visible

* goto the beginning of the second message (first line, first colon)

* hit "RET"

Result: nothing happens except for "No URL at point" message

Expected result: the second message is shown/hidden

The root cause is that the HTML part has `keymap' text property set.
In particular, "RET" is bound to visit a URL at point.  The problem is
that `keymap' property affects the next character following the region
it is set to (see elisp manual [1]).  Hence, the first character of
the next message has a keymap inherited from the HTML part.

[1] http://www.gnu.org/software/emacs/elisp/html_node/Special-Properties.html