test: add vtu style plugin (#8132)

test/index.ts

@@ -1,3 +1,5 @@
+import './plugin';
+
 // promisify setTimeout
 export function later(delay = 0): Promise<void> {
   return new Promise((resolve) => {

test/plugin.ts

@@ -0,0 +1,26 @@
+/* eslint-disable max-classes-per-file */
+import { ComponentPublicInstance } from 'vue';
+import { config, VueWrapper, DOMWrapper } from '@vue/test-utils';
+
+declare module '@vue/test-utils' {
+  // eslint-disable-next-line
+  export class DOMWrapper<ElementType extends Element> {
+    style: CSSStyleDeclaration;
+  }
+
+  // eslint-disable-next-line
+  class VueWrapper<T extends ComponentPublicInstance> {
+    style: CSSStyleDeclaration;
+  }
+}
+
+const stylePlugin = (
+  wrapper: VueWrapper<ComponentPublicInstance> | DOMWrapper<Element>
+) => {
+  return {
+    style: (wrapper.element as HTMLElement).style,
+  };
+};
+
+config.plugins.DOMWrapper.install(stylePlugin);
+config.plugins.VueWrapper.install(stylePlugin);