Add testcase for invalid %cpu placeholder

testcases/024-cpu-usage-invalid-cpu/expected_output.pl

@@ -0,0 +1,7 @@
+#!/usr/bin/env perl
+
+use v5.10;
+use strict;
+use warnings;
+
+print "CPU: \n";

testcases/024-cpu-usage-invalid-cpu/i3status.conf

@@ -0,0 +1,9 @@
+general {
+        output_format = "none"
+}
+
+order += "cpu_usage"
+
+cpu_usage {
+    format = "CPU: %cpu"
+}