tests: Add torture library from socket_wrapper

Pretty much a blatant copy of the torture library from socket_wrapper,
just s/echo_srv/dns_srv/

Signed-off-by: Jakub Hrozek <jakub.hrozek@gmail.com>
tests: Add generating resolv.conf to torture

Adds a setup/teardown functions that allow a unit test to easily
generate a resolv.conf pointing to our test DNS server.

Signed-off-by: Jakub Hrozek <jakub.hrozek@gmail.com>
tests: Add a unit test for res_nquery and res_nsearch

Adds a test that connects to a test DNS server and searches it using
res_query and res_search.

Signed-off-by: Jakub Hrozek <jakub.hrozek@gmail.com>
tests: Also test res_nclose

Adds a test for res_nclose in a positive and negative situation.

Signed-off-by: Jakub Hrozek <jakub.hrozek@gmail.com>
WIP: Add faking DNS replies