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).