fix: Added mockdom.before method (#4724)

Author: andrewnester

lib/ace/test/mockdom.js

@@ -165,6 +165,9 @@ function Node(name) {
 
         return node;
     };
+    this.before = function(node) {
+        if (this.parentNode) this.parentNode.insertBefore(node, this);
+    };
     this.hasChildNodes = function() {
         return this.childNodes.length > 0;
     };