ctdb-tests: Don't use nc -d or -w options

ctdb-tests: Don't use nc -d or -w options

nmap-ncat is used in some distributions to replace netcat.  It has a
different meaning for these options.

We can get the same effect as the current combination of -d and -w by
piping a sleep process to nc.  Subsequent use of $! works because it
gets the last process in pipeline.

Note that redirecting from /dev/null doesn't work with some versions
of nc.  They just exit when they get EOF.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
Autobuild-User(master): Martin Schwenke <martins@samba.org>
Autobuild-Date(master): Fri Mar  9 12:24:13 CET 2018 on sn-devel-144