Added unit test for referenced parent selectors

Name: Parent selectors not working within mixins using (reference)
Number: #1979

test/less/import/import-reference.less

@@ -49,7 +49,7 @@
     color: red;
   }
 }
-
+//https://github.com/less/less.js/issues/2359
 @supports (something: else) {
   .class {
     something: else;
@@ -63,3 +63,12 @@
     something: else;
   }
 }
+//https://github.com/less/less.js/issues/1979
+.mixin-with-nested-selectors() {
+  .test {
+    color: red;
+    &:first-child {
+      color: blue;
+    }
+  }
+}