[DRE-maint] Bug#1020580: FTBFS: ruby3.1 needs network/DNS during tests

[Sven Mueller]

Package: ruby3.1
Version: 3.1.2-3

The package violates policy 4.9, namely:

For packages in the main archive, required targets must not attempt
network access, except, via the loopback interface, to services on the
build host that have been started by the build.

The relevant snippet from a build with restricted network (only
internal Debian mirror available):

  1) Error:
TestAddressResolve#test_socket_getnameinfo_domain_blocking:
SocketError: getnameinfo: Temporary failure in name resolution
    /<<PKGBUILDDIR>>/test/fiber/test_address_resolve.rb:272:in `getnameinfo'
    /<<PKGBUILDDIR>>/test/fiber/test_address_resolve.rb:272:in `block
(2 levels) in test_socket_getnameinfo_domain_blocking'

The test attempts to resolve example.com which our resolver returns a
temporary error for.

The interesting part is: Why does it? If I read the source correctly,
it *should* use a stub resolver? And if it doesn't use the stub: Why
would the test succeed on the Debian autobuilders?
So this might point at some more serious issue?

Cheers,
Sven