Merge branch 'vs-add-scrolly-by-shim-to-jsdom' into 'master'

Add Element.scrollBy shim for Jest+JSDOM

See merge request gitlab-org/gitlab!27766

spec/frontend/helpers/dom_shims/element_scroll_by.js

+Element.prototype.scrollBy = jest.fn();

spec/frontend/helpers/dom_shims/index.js

 import './element_scroll_into_view';
+import './element_scroll_by';
 import './form_element';
 import './get_client_rects';
 import './inner_text';