test: run emacs inside screen

test: run emacs inside screen

Before the change, emacs run in daemon mode without any visible
buffers.  Turns out that this affects emacs behavior in some
cases.  In particular, `window-end' function returns `point-max'
instead of the last visible position.  That makes it hard or
impossible to implement some tests.  The patch runs emacs in a
detached screen(1) session.  So that it works exactly as if it
has a visible window.

Note: screen terminates when emacs exits.  So the patch does not
introduce new "running processes left behind" issues.