Fixed the failure of test_valid_pfc_frame_with_snappi.py in Cisco DUT (…

…sonic-net#15002)

* Fixed the testcase as Cisco DUT generates XON frames.
This causes the PFC capture sampling to not detect a XOFF frame being capture.
Further the buffer size for capture could be small or big depending on the IXIA model capabaility
Hence using continous XOFF injection to avoid overwriting the buffer with XON frames post a XON-XOFF-XON transition

* Made !is_cisco do regular else cisco specific

tests/snappi_tests/pfc/files/helper.py

@@ -18,7 +18,8 @@
     verify_in_flight_buffer_pkts, verify_unset_cev_pause_frame_count, verify_tx_frame_count_dut, \
     verify_rx_frame_count_dut
 from tests.common.snappi_tests.snappi_test_params import SnappiTestParams
-from tests.common.snappi_tests.read_pcap import validate_pfc_frame
+from tests.common.cisco_data import is_cisco_device
+from tests.common.snappi_tests.read_pcap import validate_pfc_frame, validate_pfc_frame_cisco
 
 
 logger = logging.getLogger(__name__)
@@ -191,7 +192,7 @@ def run_pfc_test(api,
         # PFC pause frame capture is not requested
         valid_pfc_frame_test = False
 
-    if valid_pfc_frame_test:
+    if valid_pfc_frame_test and not is_cisco_device(duthost):
         snappi_extra_params.traffic_flow_config.pause_flow_config["flow_dur_sec"] = DATA_FLOW_DURATION_SEC + \
             data_flow_delay_sec + SNAPPI_POLL_DELAY_SEC + PAUSE_FLOW_DUR_BASE_SEC
         snappi_extra_params.traffic_flow_config.pause_flow_config["flow_traffic_type"] = \
@@ -245,7 +246,11 @@ def run_pfc_test(api,
 
     # Verify PFC pause frames
     if valid_pfc_frame_test:
-        is_valid_pfc_frame, error_msg = validate_pfc_frame(snappi_extra_params.packet_capture_file + ".pcapng")
+        if not is_cisco_device(duthost):
+            is_valid_pfc_frame, error_msg = validate_pfc_frame(snappi_extra_params.packet_capture_file + ".pcapng")
+        else:
+            is_valid_pfc_frame, error_msg = validate_pfc_frame_cisco(
+                                                              snappi_extra_params.packet_capture_file + ".pcapng")
         pytest_assert(is_valid_pfc_frame, error_msg)
         return