Fix flow (#7499)

ReactElement requires a generic argument now and the return function of render is a ReactTestInstance and not a ReactElement.

src/renderers/testing/ReactTestMount.js

@@ -132,8 +132,8 @@ ReactTestInstance.prototype.toJSON = function() {
 var ReactTestMount = {
 
   render: function(
-    nextElement: ReactElement
-  ): ?ReactElement<any, any, any> {
+    nextElement: ReactElement<any>
+  ): ReactTestInstance {
     var nextWrappedElement = React.createElement(
       TopLevelWrapper,
       { child: nextElement }