tests/: Make parallel tests work.

Fix parallel tests: Use port numbers from different ranges for
each test case.
* tests/client-server.scm, tests/server-client.scm: Update.
* tests/sssh-ssshd.scm: Update.  Set port for ssshd.
* NEWS: Update.
Don't include .x files in the distribution.

* ssh/Makefile.am (AM_CPPFLAGS): Add pathes to includes.
  (snarfcppopts): Add `AM_CPPFLAGS'.
  (nodist_noinst_HEADERS): Remove.