tests: Fix isis_topo1_vrf to wait a tiny bit for zebra route install

tests: Fix isis_topo1_vrf to wait a tiny bit for zebra route install

During repeated runs I am seeing this test fail to run successfully.
Upon inspecting the output:
            {
              "prefix":"10.0.10.0/24",
              "prefixLen":24,
              "protocol":"isis",
              "vrfId":6,
              "vrfName":"r1-cust1",
              "selected":true,
              "destSelected":true,
              "distance":115,
              "metric":10,
              "queued":true,

We can see that the route is still queued.  Under heavy system
load and not ensuring that isis has time to send the route to
zebra and for zebra to install the route, this test can fail.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>