tests: add a topotest to verify nhrp shortcuts in a redundant nhs topology

tests: add a topotest to verify nhrp shortcuts in a redundant nhs topology

Contains 2 testcases. The first does a basic configuration/connectivity.
The second testcase initiates a shortcut through the primary NHS,
verifies shortcut routes are installed. Primary NHS interface brought
down and verify that the shortcut is not impacted. Finally verify that
after the shortcut expires, it is able to be re-established via a backup
NHS.

Signed-off-by: dleroy <dleroy@labn.net>
(cherry picked from commit a7037ab23407d5667385f9be096e347caa0c94ad)