ctdb-tests: Enable job control when keeping stdin open

ctdb-tests: Enable job control when keeping stdin open

POSIX says:

  If job control is disabled (see set, -m), the standard input for an
  asynchronous list, before any explicit redirections are performed,
  shall be considered to be assigned to a file that has the same
  properties as /dev/null. This shall not happen if job control is
  enabled. In all cases, explicit redirection of standard input shall
  override this activity.

ctdbd is backgrounded at startup, so the above causes stdin to be
redirected from /dev/null.  Enable job control to work around this.

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): Tue Jan 28 11:24:35 UTC 2020 on sn-devel-184