tests: add remote LFA unit tests

tests: add remote LFA unit tests

Extend the existing SPF unit testing infrastructure so that it can
test RLFA as well.

These new unit tests are useful to test the RLFA PQ node
computation on several different network topologies in a timely
manner. Artificial LDP labels (starting from 50000) are used to
activate the computed RLFAs.

It's worth mentioning that the computed backup routing tables
contain both local LFAs and remote LFAs, as running RLFA separately
isn't possible.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>