Merge bitcoin/bitcoin#24205: init, test: improve network reachability test coverage and safety

58a14795b8 test: passing -onlynet=onion with -onion=0/-noonion raises expected init error (Jon Atack)
7000f66d36 test: passing -onlynet=onion without -proxy/-onion raises expected init error (Jon Atack)
8332e6e4cf test: passing invalid -onion raises expected init error (Jon Atack)
d5edb08708 test: passing invalid -proxy raises expected init error (Jon Atack)
bd57dcbaf2 test: hoist proxy out of 2 network loops in feature_proxy.py (Jon Atack)
afdf2de282 test: add CJDNS to LimitedAndReachable_Network unit tests (Jon Atack)
2b7a8180a9 net, init: assert each network reachability is true by default (Jon Atack)

Pull request description:

  Adds missing network reachability test coverage and an assertion during init, noticed while reviewing #22834:

  - assert during init that each network reachability is  true by default
  - add CJDNS to the `LimitedAndReachable_Network` unit tests
  - hoist proxy out of two network loops in feature_proxy.py
  - test that passing invalid `-proxy` raises expected init error
  - test that passing invalid `-onion` raises expected init error
  - test that passing `-onlynet=onion` without `-proxy` and `-onion` raises expected init error
  - test that passing `-onlynet=onion` with `-onion=0` and with `-noonion` raises expected init error

ACKs for top commit:
  vasild:
    ACK 58a14795b8
  brunoerg:
    ACK 58a14795b8
  dongcarl:
    Code Review ACK 58a14795b8

Tree-SHA512: bdee6dd0c12bb63591ce7c9321fe77b509ab1265123054e774adc38a187746dddafe1627cbe89e990bcc78b45e194bfef8dc782710d5b217e2e2106ab0158827

src/init.cpp

@@ -1302,6 +1302,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         }
         for (int n = 0; n < NET_MAX; n++) {
             enum Network net = (enum Network)n;
+            assert(IsReachable(net));
             if (!nets.count(net))
                 SetReachable(net, false);
         }