Add a test for 400ZR transceiver low power mode setting. (openconfig#…

…2621)

* Add a test for 400ZR transceiver low power mode setting.

feature/platform/transceiver/zr_low_power_mode_test/README.md

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+# TRANSCEIVER-13: Configuration: 400ZR Transceiver Low Power Mode Setting.
+
+## Summary
+
+Validate 400ZR transceiver is able to move to low power consumption mode when
+the interface/config/enabled state is set to "False"
+
+**NOTE:** The Module Power Mode dictates the maximum electrical power that the
+module is permitted to consume while operating in that Module Power Mode.
+The Module Power Mode is a function of the state of the Module State Machine.
+Two Module Power Modes are defined:
+  * In Low Power Mode (characteristic of all MSM steady states except
+    ModuleReady) the maximum module power consumption is defined in the form
+    factor-specific hardware specification.
+  * In High Power Mode (characteristic of the MSM state ModuleReady) the
+    implementation dependent maximum module power consumption is advertised in
+    the MaxPower Byte 00h:201. More details in the CMIS link below.
+
+Link to CMIS:
+https://www.oiforum.com/wp-content/uploads/CMIS5p0_Third_Party_Spec.pdf
+
+## Procedure
+
+*   Connect two ZR optics using a duplex LC fiber jumper such that TX
+    output power of one is the RX input power of the other module.
+*   To establish a point to point ZR link ensure the following:
+      * Both transceivers state is enabled.
+      * Both transceivers are set to a valid target TX output power
+        example -10 dBm.
+      * Both transceivers are tuned to a valid centre frequency
+        example 193.1 THz.
+## Testbed Type
+*   Typical test setup for this test is a DUT1 with 2 ports to 2 ATE ports or 2
+    ports to a second DUT2. For most tests this setup should be sufficient.
+    Ref: [Typical ATE<>DUT Test bed](https://github.com/openconfig/featureprofiles/blob/main/topologies/atedut_2.testbed)
+*   A and Z ends of  the link should have same 400ZR PMD. For this test a
+    single DUT ZR port connected to a single ZR ATE port is also sufficient. 
+
+Once the ZR link is estabished proceed with the following:
+*   Verify that the following ZR transceiver OC path when set to False is able
+    to move to the low power mode as defined in the CMIS.
+
+    *   /interfaces/interface/config/enabled
+
+*   In low power mode the module's Management interface should be available,
+    entire paged management memory should be accessible. During this state,
+    the host may configure the module using the management interface to read
+    from and write to the management Memory Map.
+
+*   The Data Path State of all lanes is still DPDeactivated in the ModuleLowPwr
+    state.
+
+*   With module in low power mode verify that the module is still able to
+    report inventory information through the following OC paths.
+
+    *   /platform/components/component/state/serial-no
+    *   /platform/components/component/state/part-no
+    *   /platform/components/component/state/type
+    *   /platform/components/component/state/description
+    *   /platform/components/component/state/mfg-name
+    *   /platform/components/component/state/mfg-date
+    *   /platform/components/component/state/hardware-version
+    *   /platform/components/component/state/firmware-version
+
+*  With module in low power mode verify that the module laser is squelched
+   and it is no longer able to report output-power under the following OC
+   paths.
+    *   /components/component/optical-channel/state/output-power/instant
+    *   /components/component/optical-channel/state/output-power/avg
+    *   /components/component/optical-channel/state/output-power/min
+    *   /components/component/optical-channel/state/output-power/max
+
+*   Set the interface/config/enabled state to True 
+
+    *   Verify module is able to transition into High Power Mode.
+    *   In this state module is still able to report all the inventory
+        information as verified above.
+    *   In this state verify module is able to report a valid output power
+        through the following OC paths as provisioned earlier. 
+
+        *   /components/component/optical-channel/state/output-power/instant
+        *   /components/component/optical-channel/state/output-power/avg
+        *   /components/component/optical-channel/state/output-power/min
+        *   /components/component/optical-channel/state/output-power/max
+
+    *   Verify the ZR optics TX output power telemetry values are updated to
+        the value in the normal range again.
+        *  Typical min/max value range for TX Output Power -13 to -9 dbm.
+        *  values must always be of type decimal64.
+        *  When link interfaces are in down state 0 must be reported as a valid
+           value.
+
+    *   When the modules or the devices are still in a boot stage, they must not
+        stream any invalid string values like "nil" or "-inf" until valid values
+        are available for streaming.
+
+## Config Parameter coverage
+
+*   /interfaces/interface/config/enabled
+
+## Telemetry Parameter coverage
+
+*   /platform/components/component/state/serial-no
+*   /platform/components/component/state/part-no
+*   /platform/components/component/state/type
+*   /platform/components/component/state/description
+*   /platform/components/component/state/mfg-name
+*   /platform/components/component/state/mfg-date
+*   /platform/components/component/state/hardware-version
+*   /platform/components/component/state/firmware-version
+*   /components/component/optical-channel/state/output-power/instant
+*   /components/component/optical-channel/state/output-power/avg
+*   /components/component/optical-channel/state/output-power/min
+*   /components/component/optical-channel/state/output-power/max