resolved: make sure idn2 conversions are roundtrippable

While working on the gateway→_gateway conversion, I noticed that
libidn2 strips the leading underscore in some names.
https://gitlab.com/libidn/libidn2/issues/30 was resolved in
https://gitlab.com/libidn/libidn2/commit/05d753ea69e2308cd02436d0511f4b844071dc79,
which disabled "STD3 ASCII rules" by default, i.e. disabled stripping
of underscores. So the situation is that with previously released libidn2
versions we would get incorrect behaviour, and once new libidn2 is released,
we should be OK.

Let's implement a simple test which checks that the name survives the
roundtrip, and if it doesn't, skip IDN resolution. Under old libidn2 this will
fail in more cases, and under new libidn2 in fewer, but should be the right
thing to do also under new libidn2.