test: don't bail out of `run_emacs' too early when missing prereqs

When running the Emacs tests in verbose mode, only the first missing
prereq is reported because the `run_emacs' function is short-circuited
early:

  #+begin_example
    emacs: Testing emacs interface
     missing prerequisites: [0]  emacs(1)
     skipping test: [0]  Basic notmuch-hello view in emacs
     SKIP   [0]  Basic notmuch-hello view in emacs
  #+end_example

This can lead to situations reminiscent of "dependency hell", so instead
of returning based on each individual `test_require_external_prereq's exit
status, we now do so only after checking all the prereqs:

  #+begin_example
    emacs: Testing emacs interface
     missing prerequisites: [0]  dtach(1) emacs(1) emacsclient(1)
     skipping test: [0]  Basic notmuch-hello view in emacs
     SKIP   [0]  Basic notmuch-hello view in emacs
  #+end_example

Also added missing prereq for dtach(1).