Adds new testinfra test for ossec

We verify that Xenial uses sysv script, and Focal is using
the ossec.service file to start the service in the mon server.

molecule/testinfra/mon/test_ossec_server.py

@@ -5,6 +5,7 @@
 
 securedrop_test_vars = testutils.securedrop_test_vars
 testinfra_hosts = [securedrop_test_vars.monitor_hostname]
+python_version = securedrop_test_vars.python_version
 
 
 def test_ossec_connectivity(host):
@@ -21,6 +22,20 @@ def test_ossec_connectivity(host):
         assert c == desired_output
 
 
+def test_service_start_style(host):
+    """
+    Ensures to check that the service starts via sysv scrip in Xenial, and
+    uses the systemd service in Focal.
+    """
+    if python_version == "3.8":
+        value = "/etc/systemd/system/ossec.service"
+    else:
+        value = "/etc/init.d/ossec"
+    with host.sudo():
+        c = host.check_output("systemctl status ossec")
+        assert value in c
+
+
 # Permissions don't match between Ansible and OSSEC deb packages postinst.
 @pytest.mark.xfail
 @pytest.mark.parametrize('keyfile', [