Add unit tests for transferrable links

[tahini]

The "transferrable links" concept is a hack to allow to "fake" pedestrian links that don't exist in osrm.

They should not be treated as any other mode of transit. The current behavior is unclear? Users are reporting bugs related to transferrable links, could they come from trRouting? We don't know, because we don't have tests to make sure they work correctly!

So let's add tests to make sure there is never a regression again (if regression there is...)

It is not expected to have any waiting time for those links, it should just take the first "vehicle" to pass by.