test-network: save and restore timezone

Several DHCP client tests change the system timezone.
Let's save the current timezone at the beginning, and restore it with
the saved value at the end.

test/test-network/systemd-networkd-tests.py

@@ -71,6 +71,7 @@ protected_links = {
 saved_routes = None
 saved_ipv4_rules = None
 saved_ipv6_rules = None
+saved_timezone = None
 
 def rm_f(path):
     if os.path.exists(path):
@@ -466,6 +467,17 @@ def flush_l2tp_tunnels():
         else:
             print(f'Cannot remove L2TP tunnel {tid}, ignoring.')
 
+def save_timezone():
+    global saved_timezone
+    r = run('timedatectl show --value --property Timezone')
+    if r.returncode == 0:
+        saved_timezone = r.stdout.rstrip()
+        print(f'### Saved timezone: {saved_timezone}')
+
+def restore_timezone():
+    if saved_timezone:
+        call(f'timedatectl set-timezone {saved_timezone}')
+
 def read_link_attr(*args):
     with open(os.path.join('/sys/class/net', *args), encoding='utf-8') as f:
         return f.readline().strip()
@@ -615,6 +627,7 @@ def setUpModule():
     save_existing_links()
     save_routes()
     save_routing_policy_rules()
+    save_timezone()
 
     drop_in = [
         '[Unit]',
@@ -711,6 +724,8 @@ def tearDownModule():
     clear_network_units()
     clear_networkd_conf_dropins()
 
+    restore_timezone()
+
     rm_rf('/run/systemd/system/systemd-networkd.service.d')
     rm_rf('/run/systemd/system/systemd-resolved.service.d')
     rm_rf('/run/systemd/system/systemd-udevd.service.d')