using `std::env::set_var` to set or change the value of either
DNS_TEST_SUBJECT or DNS_TEST_PEER is A Bad Idea, specially so when
tests are running in parallel
we can't forbid the use of `env::set_var` _but_ at least we can ensure
that even in its presence the return value of `dns_test::{subject,peer}`
will not change
this is accomplished using a "lazy" static variable that gets
initialized at most once during the lifetime of the process instead of
reading the env var each time `{subject,peer}` is called
to better convey the fact that the return value of `{subject,peer}`
won't change, we present them as static variables instead
