Tests for:
- inheriting a receiver from the default route
- overriding the receiver when the parent route has not set one
- inheriting a receiver from a parent route that's not the default
Adds a route which matches on a `group_by` label which inherits the
default receiver and also has a child route, matching on an `env` label,
which overrides the receiver.
To be sure of edge case behaviour, we test inheriting the receiver from
a child route. Hopefully no one would actually implement this edge case
in reality as it's overly complex but we test anyway to be sure that it
doesn't trigger unexpected behaviour.
Matt Bostock
Fabian Reinartz
beorn7