Add test for the 'focus mode_toggle' issue (#487)

testcases/t/74-regress-focus-toggle.t

@@ -0,0 +1,17 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Regression: Checks if i3 still lives after using 'focus mode_toggle' on an
+# empty workspace. This regression was fixed in
+# 0848844f2d41055f6ffc69af1149d7a873460976.
+#
+use i3test;
+use v5.10;
+
+my $tmp = fresh_workspace;
+
+cmd 'focus mode_toggle';
+
+does_i3_live;
+
+done_testing;