feat(wrapper): allow destroy() method to work with functional components

Currently when calling `wrapper.destroy()`, only non-functional components are removed from the DOM.

This PR allows functional components to be removed from the DOM as well.

Author: tmorehouse

packages/test-utils/src/wrapper.js

@@ -124,16 +124,19 @@ export default class Wrapper implements BaseWrapper {
    * Calls destroy on vm
    */
   destroy(): void {
-    if (!this.isVueInstance()) {
-      throwError(`wrapper.destroy() can only be called on a Vue instance`)
+    if (!this.isVueInstance() || !this.isFunctionalComponent) {
+      throwError(`wrapper.destroy() can only be called on a Vue instance or functional component`)
     }
 
     if (this.element.parentNode) {
       this.element.parentNode.removeChild(this.element)
     }
-    // $FlowIgnore
-    this.vm.$destroy()
-    throwIfInstancesThrew(this.vm)
+
+    if (!this.isFunctionalComponent) {
+      // $FlowIgnore
+      this.vm.$destroy()
+      throwIfInstancesThrew(this.vm)
+    }
   }
 
   /**