Add UT for orchagent watchdog (sonic-net#8306)

### Description of PR
Add UT for orchagent watchdog.

Summary:
SWSS service will add watchdog mechanism to generate keepalive message, and generate alert when swss have issue.
This PR will add new UT to cover the watchdog mechanism.

### Type of change

- [ ] Bug fix
- [ ] Testbed and Framework(new/improvement)
- [x] Test case(new/improvement)


### Back port request
- [ ] 201911
- [ ] 202012
- [ ] 202205

### Approach
#### What is the motivation for this PR?
Add new UT to test and protect watchdog mechanism from code change.

#### How did you do it?
Pause orchagent service with 'kill -stop' command and check if the watchdog can send alert.

#### How did you verify/test it?
Manually test new UT.
Pass PR validation.

#### Any platform specific information?
No

#### Supported testbed topology if it's a new test case?
Any

### Documentation
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(If it's a new feature, new test case)
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Link to the wiki page?
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.azure-pipelines/pr_test_scripts.yaml

@@ -67,6 +67,7 @@ t0:
   - test_interfaces.py
   - test_procdockerstatsd.py
   - database/test_db_scripts.py
+  - system_health/test_watchdog.py
 
 t0-2vlans:
   - dhcp_relay/test_dhcp_relay.py

tests/system_health/test_watchdog.py

@@ -0,0 +1,63 @@
+import logging
+import pytest
+import time
+from tests.common.helpers.assertions import pytest_assert
+
+pytestmark = [
+    pytest.mark.disable_loganalyzer,
+    pytest.mark.topology('any')
+]
+
+logger = logging.getLogger(__name__)
+
+SLEEP_TIME = 10
+
+
+@pytest.fixture
+def pause_orchagent(duthost):
+    # find orchagent pid
+    pid = duthost.shell(
+                    r"pgrep orchagent",
+                    module_ignore_errors=True)['stdout']
+    logger.info('Get orchagent pid: {}'.format(pid))
+
+    # pause orchagent and clear syslog
+    duthost.shell(r"sudo kill -STOP {}".format(pid), module_ignore_errors=True)
+    duthost.shell(r"sudo truncate -s 0 /var/log/syslog", module_ignore_errors=True)
+
+    yield
+
+    # resume orchagent and clear syslog
+    duthost.shell(r"sudo kill -CONT {}".format(pid), module_ignore_errors=True)
+    duthost.shell(r"sudo truncate -s 0 /var/log/syslog", module_ignore_errors=True)
+
+
+def test_orchagent_watchdog(duthosts, enum_rand_one_per_hwsku_hostname, pause_orchagent):
+    duthost = duthosts[enum_rand_one_per_hwsku_hostname]
+
+    result = duthost.shell(
+                        r"docker exec -i swss sh -c 'test -f /etc/supervisor/watchdog_processes && echo exist'",
+                        module_ignore_errors=True)['stdout']
+    logger.info('Check watchdog exist: {}'.format(result))
+    if result != 'exist':
+        pytest.skip("Skip orchagent watchdog test.")
+
+    # wait watchdog emit alert, orchagent watchdog timeout is 60 seconds
+    WATCHDOG_TIMEOUT = 120
+    current_attempt = 0
+    while (True):
+        time.sleep(SLEEP_TIME)
+        alert = duthost.shell(
+                            r"sudo cat /var/log/syslog | grep 'is stuck in namespace'",
+                            module_ignore_errors=True)['stdout']
+        logger.info('Get alert from host: {}'.format(alert))
+        if "orchagent" in str(alert):
+            return
+        else:
+            # orchagent watchdog timeout is 60 seconds
+            if current_attempt >= WATCHDOG_TIMEOUT/SLEEP_TIME:
+                pytest_assert(
+                            False,
+                            "orchagent watchdog did not been trigger after {} seconds".format(WATCHDOG_TIMEOUT))
+            else:
+                current_attempt += 1