Add dummy ARP cache for screenshots

screenshots/arp

@@ -0,0 +1,3 @@
+IP address       HW type     Flags       HW address            Mask     Device
+192.168.2.100    0x1         0x2         62:6f:a3:f8:3e:ef     *        eth0
+192.168.2.101    0x1         0x2         a3:6f:7f:3a:9d:ff     *        eth0

screenshots/run-for-screenshot.bash

@@ -2,6 +2,8 @@
 
 set -euo pipefail
 
+DEVICES_FILE="${1:-"$(git rev-parse --show-toplevel)/screenshots/devices.json"}"
+
 variables=(
     # Run app with default settings: Force the in-memory gsettings backend to
     # block access to standard Gtk settings, and tell Adwaita not to access
@@ -14,4 +16,5 @@ variables=(
 )
 
 exec env "${variables[@]}" cargo run -- \
-    --devices-file "$(git rev-parse --show-toplevel)/screenshots/devices.json"
+    --devices-file "${DEVICES_FILE}" \
+    --arp-cache-file "$(git rev-parse --show-toplevel)/screenshots/arp"