Add libnetd_resolv_test for dns internal tests

Separate libnetddns unit tests from netd_integration_test. As these
unit tests are for libnetddns only, put them in libnetd_resolv_test
which will be specific for the internal tests. Also let runtest.sh
test libnetd_resolv_test.

Bug: 113628807
Test: as follows
    - built, flashed, booted
    - system/netd/tests/runtests.sh and libnetd_resolv_test passed

Change-Id: I4342ca94510d554a4817d7c10d463a33719c4bd1