[tests] [asan] add graceful stop flag (sonic-net#2347)

- What I did
Added a new flag to DVS tests

- Why I did it
Currently, when running the tests with ASAN-enabled image, leak reports are not generated. The reason is that dvs.destroy() (via 'ctn.remove(force=True)') uses SIGKILL to stop the container. To address this, a new flag is added.
When the new flag is set, the swss processes are gracefully stopped (via SIGTERM).
So ASAN reports can be generated as a result of DVS tests run

- How I verified it
Run the tests with --graceful-stop, observe that swss processes are stopped via SIGTERM

Signed-off-by: Yakiv Huryk <yhuryk@nvidia.com>

tests/conftest.py

@@ -89,6 +89,11 @@ def pytest_addoption(parser):
                      default="traditional",
                      help="Buffer model")
 
+    parser.addoption("--graceful-stop",
+                     action="store_true",
+                     default=False,
+                     help="Stop swss before stopping a conatainer")
+
 
 def random_string(size=4, chars=string.ascii_uppercase + string.digits):
     return "".join(random.choice(chars) for x in range(size))
@@ -1730,6 +1735,8 @@ def manage_dvs(request) -> str:
     max_cpu = request.config.getoption("--max_cpu")
     buffer_model = request.config.getoption("--buffer_model")
     force_recreate = request.config.getoption("--force-recreate-dvs")
+    graceful_stop = request.config.getoption("--graceful-stop")
+
     dvs = None
     curr_dvs_env = [] # lgtm[py/unused-local-variable]
 
@@ -1778,6 +1785,8 @@ def update_dvs(log_path, new_dvs_env=[]):
 
     yield update_dvs
 
+    if graceful_stop:
+        dvs.stop_swss()
     dvs.get_logs()
     dvs.destroy()