tests: net: dns: Add tests for catching malformed packets

Add more tests to verify that we discard malformed packets.
In order to simplify the testing, separate message validation to
dns_validate_msg() function in resolve.c. Allow that function to
be called from unit test. This way we can construct invalid DNS
messages in unit test and verify that they are discarded when
needed.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>