Make helper plug gap in JSDOM's DOM API

Errors fired from JSDOM showed it doesn't support
`window.scrollTo` (or `window.scroll` for that
matter).

This stubs it, to the extent that our use (jQuery
really) of it works.

tests/javascripts/support/helpers.js

@@ -228,6 +228,7 @@ class WindowMock {
     };
     this._jest = jest;
     this._setSpies();
+    this._plugJSDOM();
   }
 
   get top () {
@@ -260,6 +261,27 @@ class WindowMock {
 
   }
 
+  _plugJSDOM () {
+
+    const self = this;
+
+    // JSDOM doesn't support .scrollTo
+    window.scrollTo = function () {
+      let y;
+
+      // data sent as props in an object
+      if (arguments.length === 1) {
+        y = arguments[0].y;
+      } else {
+        y = arguments[1];
+      }
+
+      self.scrollTo(y);
+
+    };
+
+  }
+
   setHeightTo (height) {
 
     window.innerHeight = height;