add isis drain test (openconfig#2052)

* add isis drain test

This code is a Contribution to the OpenConfig Feature Profiles project ("Work") made under the Google Software Grant and Corporate Contributor License Agreement ("CLA") and governed by the Apache License 2.0. No other rights or licenses in or to any of Nokia’s intellectual property are granted for any other purpose. This code is provided on an "as is" basis without any warranties of any kind.

feature/experimental/isis/otg_tests/isis_drain_test/README.md

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+# RT-2.12: IS-IS Drain Test
+
+## Summary
+
+Ensure that IS-IS metric change can drain traffic from a DUT trunk interface
+
+## Procedure
+* Connect three ATE ports to the DUT
+* Port-2 and port-3 each makes a one-member trunk port with the same ISIS metric 10 configured for the trunk interfaces (trunk-2 and trunk-3).  
+* Configure a destination network-a connected to trunk-2 and trunk-3.
+* Send 10K IPv4 traffic flows from ATE port-1 to network-a. Validate that traffic is going via trunk-2 and trunk-3 and there is no traffic loss
+* Change the ISIS metric of trunk-2 to 1000 value. Validate that 100% of the traffic is going out of only trunk-3 and there is no traffic loss.
+* Revert back the ISIS metric on trunk-2. Validate that the traffic is going via both trunk-2 and trunk-3, and there is no traffic loss.
+
+## Config Parameter Coverage
+
+## Telemetry Parameter Coverage
+
+## Protocol/RPC Parameter Coverage
+
+*   IS-IS
+    *   LSP
+        *   TLV 22 metric field.
+
+## Minimum DUT Platform Requirement
+
+vRX