tests/net/dns: Add the test case for the DNS low-level routines

This patch adds the test case for the DNS API low-level routines.

The following routines are evaluated by this test case:

- dns_msg_pack_qname
- dns_msg_pack_query
- dns_unpack_header_id
- dns_header_qr
- dns_header_opcode
- dns_header_aa
- dns_header_tc
- dns_header_rd
- dns_header_ra
- dns_header_z
- dns_header_rcode
- dns_header_qdcount
- dns_header_ancount
- dns_header_nscount
- dns_header_arcount
- dns_unpack_query_qclass
- dns_unpack_query_qtype

Next iterations will consider IPv6 and DNS RR coverage.

Jira: ZEP-1499

Change-Id: I4ef46203e6fec57c6fe5a5db8132ed140b412fc2
Signed-off-by: Flavio Santes <flavio.santes@intel.com>