This expands the migration test quite a bit. It now tests alll the way
until retirement of the original path, for both port-only
(`rebinding_port`) and address-and-port (`rebinding_address_and_port`)
changes.

`rebinding_address_and_port` is succeeding, but `rebinding_port` is
currently failing. That's because we treat it differently for some
reason. If we replace `Paths::find_path_with_rebinding` with
`Paths::find_path`, i.e., do proper path validation when only the port
changes, the test succeeds.

Leaving this out in case I'm missing something about the intent of the
difference.

We need to do a path challenge even if only the remote port changes.

Signed-off-by: Lars Eggert <[email protected]>