test: add two emacs tests for show mode refresh

The first test tests that the notmuch-show-refresh-view function
produces the exact same output for an unmodified show buffer.  This
test should pass since the relevant functionality has already been
applied.

The second test tests show refresh for a show buffer that has been
modified by navigation and message visibility toggling.  Ideally
refresh-view should preserve this state of the notmuch-show buffer.
Unfortunately it currently does not, so this test is know to be broken
and is marked as such.