Lookup channels by node id instead of endpoints in test code

[pwojcikdev]

Right now we assume we can lookup channels by their associated endpoint, however this is only because we always establish two channels per peer, one inbound and one outbound. This PR is preparation for future changes introducing bidirectional channels where one endpoint uses a random ephemeral port.

There are also a few smaller fixes that I noticed when going through code that didn't seem worthwhile to have their own PR.

TODO: It is probably a good idea to replace looking up channels by node id everywhere, but a lot of those usages are left in legacy UDP code, so this needs to wait after that part is removed.