Adding capture group test case

test/css/functions.css

@@ -9,6 +9,7 @@
 #built-in {
   escaped: -Some::weird(#thing, y);
   gsub: "Hello, World!";
+  gsub-captured: "This is a new string.";
   lighten: #ffcccc;
   darken: #330000;
   saturate: #203c31;

test/less/functions.less

@@ -13,6 +13,7 @@
   @r: 32;
   escaped: e("-Some::weird(#thing, y)");
   gsub: gsub("Hello, Foo.", "Foo\.$", "World!");
+  gsub-captured: gsub("This is a string.", "(string)\.$", "new $1.");
   lighten: lighten(#ff0000, 40%);
   darken: darken(#ff0000, 40%);
   saturate: saturate(#29332f, 20%);