`p2p_seednode.py` would try to connect to `0.0.0.1` and `0.0.0.2` as
seed nodes. This sends outbound TCP packets on a non-loopback interface
to the default router.
Configure an unavailable proxy for all executions of `bitcoind` during
this test. Also change `0.0.0.1` and `0.0.0.2` because connecting to
them would skip the `-proxy=` setting because for such an address:
* `CNetAddr::IsLocal()` is true, thus
* `CNetAddr::IsRoutable()` is false, thus
* `CNetAddr::GetNetwork()` is `NET_UNROUTABLE`, even though
`CNetAddr::m_net` is `NET_IPV4`.
This speeds up the execution time of `p2p_seednode.py`
from 12.5s to 2.5s.
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