yanger: handle all frr uptime formats

FRR prints uptime different based on how long it is. Prior to this
commit yanger expected uptime to be on the format HH:MM:SS which it is
as long as it's shorter than a day. If it's longer the format changes,
this commit handles this.

The commit also adds unit test data triggering the different formats
(<day, <week, >week).

Side note:
cli-pretty could be made to represent the data on "FRR format" again,
which is perhaps more human friendly.

Signed-off-by: Richard Alpe <richard@bit42.se>

test/case/cli/cli-output/show-routes-ipv4.txt

@@ -6,6 +6,6 @@
    10.0.13.0/30       110/2000 e5               ospfv2    0h0m0s
 >* 10.0.13.0/30            0/0 e5               direct    0h0m0s
    10.0.23.0/30          110/1 e6               ospfv2    0h0m0s
->* 10.0.23.0/30            0/0 e6               direct    0h0m0s
-   192.168.3.0/24        110/1 e2               ospfv2    0h0m0s
->* 192.168.3.0/24          0/0 e2               direct    0h0m0s
+>* 10.0.23.0/30            0/0 e6               direct  20h10m5s
+   192.168.3.0/24        110/1 e2               ospfv2  133h5m0s
+>* 192.168.3.0/24          0/0 e2               direct  205h0m0s

test/case/cli/system-output/vtysh-ip4-route.json

@@ -228,7 +228,7 @@
       "internalNextHopActiveNum":1,
       "nexthopGroupId":23,
       "installedNexthopGroupId":23,
-      "uptime":"00:00:00",
+      "uptime":"20:10:05",
       "nexthops":[
         {
           "flags":3,
@@ -256,7 +256,7 @@
       "internalNextHopNum":1,
       "internalNextHopActiveNum":1,
       "nexthopGroupId":33,
-      "uptime":"00:00:00",
+      "uptime":"5d13h05m",
       "nexthops":[
         {
           "flags":1,
@@ -286,7 +286,7 @@
       "internalNextHopActiveNum":1,
       "nexthopGroupId":17,
       "installedNexthopGroupId":17,
-      "uptime":"00:00:00",
+      "uptime":"01w1d13h",
       "nexthops":[
         {
           "flags":3,