[test_reboot]Wait a few seconds before checking the reboot-cause (son…

…ic-net#1367)

In the reboot cause test it is assumed that the reboot-cause is available as soon as the critical services ready. However, PR Delay process-reboot-cause service until network connection is stable sonic-net#4003 introduces an extra amount of seconds and fails the assumption.

tests/platform/test_reboot.py

@@ -84,7 +84,7 @@ def check_reboot_cause(dut, reboot_cause_expected):
     reboot_cause_got = output["stdout"]
     logging.debug("show reboot-cause returns {}".format(reboot_cause_got))
     m = re.search(reboot_cause_expected, reboot_cause_got)
-    assert m is not None, "got reboot-cause %s after rebooted by %s" % (reboot_cause_got, reboot_cause_expected)
+    return m is not None
 
 
 def reboot_and_check(localhost, dut, interfaces, reboot_type=REBOOT_TYPE_COLD, reboot_helper=None, reboot_kwargs=None):
@@ -139,7 +139,8 @@ def reboot_and_check(localhost, dut, interfaces, reboot_type=REBOOT_TYPE_COLD, r
     check_critical_services(dut)
 
     logging.info("Check reboot cause")
-    check_reboot_cause(dut, reboot_cause)
+    assert wait_until(120, 20, check_reboot_cause, dut, reboot_cause), \
+        "got reboot-cause failed after rebooted by %s" % reboot_cause
 
     if reboot_ctrl_dict[reboot_type]["test_reboot_cause_only"]:
         logging.info("Further checking skipped for %s test which intends to verify reboot-cause only".format(reboot_type))