tests: Add a simple DNS server

tests: Add a simple DNS server

This DNS server will be usable for testing. Currently only supports A
records.

The server responds to any A query with the value of RWRAP_TEST_A_REC
environment variable. If undefined, the server always replies with
127.0.10.10.

Please note the server does *not* perform many security checks and is
completely unsuitable outside a test suite! Check out servers like
dnsmasq for anything more complex.

Signed-off-by: Jakub Hrozek <jakub.hrozek@gmail.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>