Simplify TTY allocation.

Simplify TTY allocation.

The 'process-tty-name' already provides the TTY name, we don't have
interrogate the TTY host.

* test/src/process-tests.el
(process-tests/fd-setsize-no-crash/make-serial-process): Use
'process-tty-name' instead of having the TTY host print its TTY
name.  Check whether TTY names are unique.
(process-tests--new-pty, process-tests--with-temp-file): Remove;
no longer used.