https://github.com/ooni/probe-cli/pull/421 was wrong because we need a more rich interface for quic-go to call ReadMsgUDP. With this commit, we use such an interface: OOBCapablePacketConn. Still part of https://github.com/ooni/probe/issues/1505.
Part of https://github.com/ooni/probe/issues/1505
I needed to add some tests as integration tests due to circular imports, but this is ~fine because we quite likely want many integration tests in the errorsx package anyway. Part of https://github.com/ooni/probe/issues/1505.