test: add a mock ModemManager for basic coverage of sd-networkd's integration

Just the minimal setup and test case required to cover
https://github.com/systemd/systemd/issues/41389 for now, can be
expanded in the future

Boring boilerplate is bot-made, don't @ me

Co-developed-by: Claude Opus 4.6 noreply@anthropic.com

test/test-network/conf/25-wwan-ipv4v6.network

@@ -0,0 +1,12 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Match]
+Name=dummy98
+
+[Network]
+LLDP=no
+LinkLocalAddressing=no
+IPv6AcceptRA=no
+
+[MobileNetwork]
+APN=internet.test
+IPFamily=both

test/test-network/conf/mock-modem-manager.conf

@@ -0,0 +1,13 @@
+<!DOCTYPE busconfig PUBLIC
+ "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
+ "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
+<busconfig>
+  <!-- Allow root to own and interact with the mock ModemManager service -->
+  <policy user="root">
+    <allow own="org.freedesktop.ModemManager1"/>
+    <allow send_destination="org.freedesktop.ModemManager1"/>
+  </policy>
+  <policy context="default">
+    <allow send_destination="org.freedesktop.ModemManager1"/>
+  </policy>
+</busconfig>