Mostly from upstream bitcoin#16898

By default all test nodes are connected in a chain. However, instead of just a single connection between each pair of nodes, we end up with up to four connections for a "middle" node (two outbound, two inbound, from each side).

This is generally redundant (tx and block relay should succeed with just a single connection) and confusing. For example, test timeouts after a call to sync_ may be racy and hard to reproduce. On top of that, the test debug.logs are hard to read because txs and block invs may be relayed on the same connection multiple times.

Fix this by inlining connect_nodes_bi in the two tests that need it, and then replace it with a single connect_nodes in all other tests.

Historic background:

connect_nodes_bi has been introduced as a (temporary?) workaround for bug dashpay#5113 and dashpay#5138, which has long been fixed in dashpay#5157 and dashpay#5662.