disable pfcwd before running qos_sai testing. (sonic-net#17393)

Description of PR
Summary:
After sonic-net/sonic-utilities#3792, we need to disable pfcwd on backplane ports.

Approach
What is the motivation for this PR?
PFCWD needs to be disabled on backplane ports when running qos_sai testing

How did you do it?
added module level fixture to disable pfcwd

co-authorized by: [email protected]

(cherry picked from commit 69034ab)

Author: sdszhang

tests/qos/qos_sai_base.py

@@ -29,6 +29,9 @@
 from tests.common.errors import RunAnsibleModuleFail
 from tests.common import config_reload
 from tests.common.devices.eos import EosHost
+from tests.common.snappi_tests.qos_fixtures import get_pfcwd_config, reapply_pfcwd
+from tests.common.snappi_tests.common_helpers import \
+        stop_pfcwd, disable_packet_aging, enable_packet_aging
 
 logger = logging.getLogger(__name__)
 
@@ -1934,6 +1937,26 @@ def dut_disable_ipv6(self, duthosts, tbinfo, lower_tor_host, swapSyncd_on_select
                     duthost, config_source='config_db', safe_reload=True, check_intf_up_ports=True,
                 )
 
+    @pytest.fixture(scope='module', autouse=True)
+    def dut_disable_pfcwd(self, duthosts):
+        switch_type = duthosts[0].facts.get('switch_type')
+        if switch_type != 'chassis-packet':
+            yield
+            return
+
+        # for packet chassis, the packet may go through backplane
+        # once tx is disabled on egress port, the continuous PFC PAUSE frame will trigger PFCWD on backplane ports
+        # to avoid the impact, we will disable it first before running the test
+        pfcwd_value = {}
+        for duthost in duthosts:
+            pfcwd_value[duthost.hostname] = get_pfcwd_config(duthost)
+            stop_pfcwd(duthost)
+            disable_packet_aging(duthost)
+        yield
+        for duthost in duthosts:
+            reapply_pfcwd(duthost, pfcwd_value[duthost.hostname])
+            enable_packet_aging(duthost)
+
     @pytest.fixture(scope='class', autouse=True)
     def sharedHeadroomPoolSize(
         self, request, duthosts, get_src_dst_asic_and_duts, tbinfo, lower_tor_host): # noqa F811