test-network: support the case that l2tp module is not supported

Hopefully fixes the issue reported at
https://github.com/systemd/systemd-centos-ci/pull/517#issuecomment-1205399798.

diff --git a/test/test-network/systemd-networkd-tests.py b/test/test-network/systemd-networkd-tests.py
index 71f284fd180331971075af846b82d43cfbbe0a62..9266441944cb30537005b1073ae9dcd737e9aaa4 100755 (executable)
--- a/test/test-network/systemd-networkd-tests.py
+++ b/test/test-network/systemd-networkd-tests.py
@@ -475,8 +475,10 @@ def flush_fou_ports():
 
 def flush_l2tp_tunnels():
     tids = []
-    output = check_output('ip l2tp show tunnel')
-    for line in output.splitlines():
+    ret = run('ip l2tp show tunnel')
+    if ret.returncode != 0:
+        return # l2tp may not be supported
+    for line in ret.stdout.splitlines():
         words = line.split()
         if words[0] == 'Tunnel':
             tid = words[1].rstrip(',')