Add test coverage for connections to device.

[robshakir]

  * (M) device/device.go
  * (M) device/device_test.go
    - add a check that gNMI updates are handled properly by the device.
  * (M) gnmit/gnmit.go
  * (M) gnmit/gnmit_test.go
    - Ensure that TLS credentials are handled properly.
    - return errors when cache update is not correctly handled.

@bstoll PTAL.

You can verify that this works using:

▶ cd $GOPATH/src/github.com/openconfig/gribigo/cmd/rtr && go build
▶ ./rtr  -cert ../../testcommon/testdata/server.cert -key ../../testcommon/testdata/server.key -alsologtostderr

To validate gNMI - using the listen address returned for gNMI, and gnmi_cli from github.com/openconfig/gnmi:

▶ ./gnmi_cli -t DUT -a 127.0.0.1:63840 -q "/" -insecure -dt s -qt o
DUT/meta/targetLeaves, 0
DUT/meta/targetLeavesUpdated, 2
DUT/meta/targetLeavesAdded, 0
DUT/meta/targetLeavesDeleted, 0
DUT/meta/connected, true
DUT/meta/sync, false
DUT/meta/latestTimestamp, 1646841252483835000
DUT/meta/targetLeavesStale, 0
DUT/meta/targetLeavesSuppressed, 0
DUT/meta/targetSize, 1975
DUT/meta/targetLeavesEmpty, 0
DUT/meta/connectedAddress, 

To validate gRIBI - using the listen address returned for gRIBI:

▶ cd $GOPATH/src/github.com/openconfig/gribigo/demo/ipv4
▶ go test -args --run --addr="127.0.0.1:63839" 
E0309 10:57:47.115324   65321 client.go:308] got error receiving message, rpc error: code = Canceled desc = grpc: the client connection is closing
PASS
ok  	github.com/openconfig/gribigo/demo/ipv4	2.710s

After this test, re-running the gNMI validation should show the AFT populated (running it during will show it being added, with -qt s rather than -qt o):

▶ ./gnmi_cli -t DUT -a 127.0.0.1:63840 -q "/" -insecure -dt s -qt o
DUT/openconfig/network-instances/network-instance/DEFAULT/afts/next-hops/next-hop/1/index, 1
DUT/openconfig/network-instances/network-instance/DEFAULT/afts/next-hops/next-hop/1/state/ip-address, 192.0.2.1
DUT/openconfig/network-instances/network-instance/DEFAULT/afts/next-hops/next-hop/1/state/index, 1
DUT/openconfig/network-instances/network-instance/DEFAULT/afts/next-hop-groups/next-hop-group/42/state/id, 42
DUT/openconfig/network-instances/network-instance/DEFAULT/afts/next-hop-groups/next-hop-group/42/id, 42
DUT/openconfig/network-instances/network-instance/DEFAULT/afts/next-hop-groups/next-hop-group/42/next-hops/next-hop/1/state/index, 1
DUT/openconfig/network-instances/network-instance/DEFAULT/afts/next-hop-groups/next-hop-group/42/next-hops/next-hop/1/index, 1
DUT/openconfig/network-instances/network-instance/DEFAULT/afts/next-hop-groups/next-hop-group/42/next-hops/next-hop/1/state/weight, 1
DUT/openconfig/network-instances/network-instance/DEFAULT/afts/ipv4-unicast/ipv4-entry/1.1.1.1/32/state/next-hop-group, 42
DUT/openconfig/network-instances/network-instance/DEFAULT/afts/ipv4-unicast/ipv4-entry/1.1.1.1/32/state/prefix, 1.1.1.1/32
DUT/openconfig/network-instances/network-instance/DEFAULT/afts/ipv4-unicast/ipv4-entry/1.1.1.1/32/prefix, 1.1.1.1/32
DUT/meta/targetSize, 3914
DUT/meta/targetLeavesEmpty, 0
DUT/meta/connectedAddress, 
DUT/meta/connected, true
DUT/meta/sync, false
DUT/meta/latestTimestamp, 1646841252483835000
DUT/meta/targetLeavesStale, 1
DUT/meta/targetLeavesSuppressed, 0
DUT/meta/targetLeaves, 3
DUT/meta/targetLeavesUpdated, 13
DUT/meta/targetLeavesAdded, 3
DUT/meta/targetLeavesDeleted, 0

PTAL and let me know if you have issues here. There's one potential fix in here, so please test on this branch. This also adds integration tests.