plugin/battery: fix tests

test/plugins/battery.plugin.bats

@@ -196,12 +196,11 @@ function setup_upower {
 	percent="$1"
 	BAT0="/org/freedesktop/UPower/devices/battery_BAT$RANDOM"
 
-
 	function upower {
 		case $1 in
 		'-e'|'--enumerate')
-			echo "$BAT0"
-			echo "/org/freedesktop/UPower/devices/mouse_hid_${RANDOM}_battery"
+			# don't just `echo` twice because `grep` will close the pipe after matching the first line...
+			echo "$BAT0"$'\n'"/org/freedesktop/UPower/devices/mouse_hid_${RANDOM}_battery"
 			;;
 		'-i'|'--show-info')
 			if [[ $2 == "$BAT0" ]]