fix: make fireEvent mouseEnter/mouseLeave work with addEventListener (testing-library#577)

Author: Zach Brogan

src/pure.js

@@ -128,8 +128,16 @@ Object.keys(dtlFireEvent).forEach(key => {
 // React event system tracks native mouseOver/mouseOut events for
 // running onMouseEnter/onMouseLeave handlers
 // @link https://github.com/facebook/react/blob/b87aabdfe1b7461e7331abb3601d9e6bb27544bc/packages/react-dom/src/events/EnterLeaveEventPlugin.js#L24-L31
-fireEvent.mouseEnter = fireEvent.mouseOver
-fireEvent.mouseLeave = fireEvent.mouseOut
+const mouseEnter = fireEvent.mouseEnter
+const mouseLeave = fireEvent.mouseLeave
+fireEvent.mouseEnter = (...args) => {
+  mouseEnter(...args)
+  return fireEvent.mouseOver(...args)
+}
+fireEvent.mouseLeave = (...args) => {
+  mouseLeave(...args)
+  return fireEvent.mouseOut(...args)
+}
 
 fireEvent.select = (node, init) => {
   // React tracks this event only on focused inputs