[Nokia-7215] Improve test_reboot stability at teardown stage (sonic-net#14654)

Nokia-7215 has low performance. After device reboot, it's critical processes may not be fully up at the time SSH is reachable. For this platform, I add wait_critical_processes at teardown stage to improve the test stability.

What is the motivation for this PR?
Improve the test_reboot stability on Nokia-7215 platform.

How did you do it?
Add wait_critical_processes at teardown stage.

How did you verify/test it?
Verified on Nokia-7215 M0 testbed.

Author: lizhijianrd

tests/platform_tests/test_reboot.py

@@ -54,6 +54,8 @@ def teardown_module(duthosts, enum_rand_one_per_hwsku_hostname,
         "Tearing down: to make sure all the critical services, interfaces and transceivers are good")
     interfaces = conn_graph_facts["device_conn"][duthost.hostname]
     wait_for_startup(duthost, localhost, delay=10, timeout=300)
+    if duthost.facts['hwsku'] in {"Nokia-M0-7215", "Nokia-7215"}:
+        wait_critical_processes(duthost)
     check_critical_processes(duthost, watch_secs=10)
     check_interfaces_and_services(duthost, interfaces, xcvr_skip_list)
     if duthost.is_supervisor_node():