This exposes private API from libnl-route-3. The only purpose is for tests to be able to access the internals. This is usable to lib/route itself, and tests that either statically or dynamically link with libnl-route-3.
The difference between nl-priv-static-route and nl-priv-dynamic-route, is that the former uses internal ABI, so it is only usable when the test statically links with lib/route. On the other hand, nl-priv-dynamic-route also works with only the public API (that is, dynamically linking with libnl-route-3).