Add passing test from less#3098

Author: matthew-dean

test/css/parse-interpolation.css

@@ -4,3 +4,8 @@ input[type=password]:focus,
 textarea:focus {
   foo: bar;
 }
+.a + .z,
+.b + .z,
+.c + .z {
+  color: blue;
+}

test/less/parse-interpolation.less

@@ -4,4 +4,12 @@
   &:focus {
     foo: bar;
   }
-} 
+} 
+
+@classes: ~".a, .b, .c";
+
+@{classes} {  
+  + .z {
+    color: blue; 
+  }
+}