tests: add OSPFv3 graceful restart topotest

tests: add OSPFv3 graceful restart topotest

Add a new topotest that features a topology with seven routers spread
across four OSPF areas:
* 1 backbone area;
* 1 regular non-backbone area (0.0.0.1);
* 1 stub area (0.0.0.2);
* 1 NSSA area (0.0.0.3).

All routers have both GR and GR helper functionality enabled in
the configuration. The test consists of restarting each router,
one at time, and checking that all forwarding planes (and LSDBs)
are kept intact during those restarts.

A successful run takes about three minutes to finish.

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