[chassis][voq] Add tests to check fabric link status and reachability. (

sonic-net#6620)

What is the motivation for this PR?
This PR adds test cases of checking fabric links.

How did you do it?
These test cases will compare the following two sets of data on a chassis:

Expected fabric link status and fabric link reachability information
Current fabric link status and reachability
How did you verify/test it?
The new voq tests read the information stored in /voq/fabric_data/ as expected value to check the fabric link status and fabric link reachability information in a system. If the information on the system is different than expected ones, the test will fail.

Any platform specific information?
The expected fabric link status /reachability information that stored in /voq/fabric_data/ is specific to SKU. Each type of cards should have its own expected data in this path.

Supported testbed topology if it's a new test case?
The new tests only runs on t2 systems.

tests/common/plugins/conditional_mark/tests_mark_conditions.yaml

@@ -1063,6 +1063,21 @@ vlan/test_vlan_ping.py:
     conditions:
       - "asic_type in ['broadcom']"
 
+#######################################
+#####             voq             #####
+#######################################
+voq/test_fabric_reach.py:
+  skip:
+    reason: "Skip test_fabric_reach on unsupported testbed."
+    conditions:
+      - "('t2' not in topo_name) or (asic_subtype not in ['broadcom-dnx']) or ('arista_7800' not in platform)"
+
+voq/test_voq_fabric_status_all.py:
+  skip:
+    reason: "Skip test_voq_fabric_status_all on unsupported testbed."
+    conditions:
+      - "('t2' not in topo_name) or (asic_subtype not in ['broadcom-dnx']) or ('arista_7800' not in platform)"
+
 #######################################
 #####             vrf             #####
 #######################################