fix ReactTestIntance type typing (#29335)

Tested with the following code:
```
component.root.find(
  node => node.type === MyButton && node.props.type === ‘submit’
)
```

types/react-test-renderer/index.d.ts

@@ -25,7 +25,7 @@ export interface ReactTestRendererTree extends ReactTestRendererJSON {
 }
 export interface ReactTestInstance {
     instance: any;
-    type: string;
+    type: ReactType;
     props: { [propName: string]: any };
     parent: null | ReactTestInstance;
     children: Array<ReactTestInstance | string>;