[dualtor] Leave icmp_responder running on dualtor-mixed testbeds (#5972)

Approach
What is the motivation for this PR?
On dualtor-mixed testbeds, leave icmp_responder running to avoid introducing unnecessary toggles.

Signed-off-by: Longxiang Lyu [email protected]

How did you do it?
Comment out the teardown to stop icmp_responder

How did you verify/test it?

Author: lolyu

tests/common/fixtures/ptfhost_utils.py

@@ -229,9 +229,8 @@ def run_icmp_responder_session(duthosts, duthost, ptfhost, tbinfo):
 
     yield
 
-    logging.info("Stop running icmp_responder")
-    ptfhost.shell("supervisorctl stop icmp_responder")
-    icmp_responder_session_started = False
+    # NOTE: Leave icmp_responder running for dualtor-mixed topology
+    return
 
 
 @pytest.fixture(scope="module", autouse=True)