Update config tests for listeners

- Group the tests by listener types rather than by options.
  This makes test layout more similar to the config spec layout.
- Introduce a new path notion - an integer in the path means an index
  in a list (TOML array), e.g. [listen, 1] is the first listener.
  The indexes start at one to be consistent with lists:nth/2
  This change allows testing config options individually, just like e.g.
  for modules.
- Add several missing tests.