test/emacs: replace the use of process-attributes with signal-process

test/emacs: replace the use of process-attributes with signal-process

In some environments (at least Hurd), process-attributes is
unimplimented and always returns nil.  This ends up causing test
failures (see e.g. id:87a9ffofsc.fsf@zancas.localnet).

Historically and according to POSIX 1003.1-2001, a signal of 0 can be
used to check the validity of a pid. This seems less heinous than
parsing the output of ps(1).