libreswan: fix DNS test
This branch fixes some problems with DNS.
First, some tests leave systemd-resolved configured on the system, while other leave plan /etc/resolv.conf configured; furthermore, some of such tests are using the @ver tag and therefore they only run on some distros or versions. The result is that the resolver state is not consistent at the beginning of a given test, and this is causing some failures. For example, the libreswan_dns currently fails under systemd-resolved.
The branch also changes some tests to either work with any resolver configuration, or to specify explicitly which resolver configuration to use.
Edited by Beniamino Galvani