CI: network - Add test for rp_filter

test/test-network/conf/25-sysctl.network

@@ -10,3 +10,4 @@ IPv4ProxyARP=yes
 IPv6ProxyNDP=yes
 IPv6AcceptRA=no
 IPv4AcceptLocal=yes
+IPv4ReversePathFilter=no

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

@@ -3250,6 +3250,7 @@ class NetworkdNetworkTests(unittest.TestCase, Utilities):
         self.check_ipv4_sysctl_attr('dummy98', 'forwarding', '1')
         self.check_ipv4_sysctl_attr('dummy98', 'proxy_arp', '1')
         self.check_ipv4_sysctl_attr('dummy98', 'accept_local', '1')
+        self.check_ipv4_sysctl_attr('dummy98', 'rp_filter', '0')
 
         copy_network_unit('25-sysctl.network.d/25-ipv6-privacy-extensions.conf')
         networkctl_reload()