Fix isTransitioning on nested navigate (react-navigation#4520)

This bug wasnt apparent until we fixed the transitioner to fully respect isTransitioning. The router did not handle this properly until now.

Enhanced a test to verify this in the future

src/routers/StackRouter.js

@@ -401,7 +401,10 @@ export default (routeConfigs, stackConfig = {}) => {
                 routeName: childRouterName,
                 key: action.key || generateKey(),
               };
-              return StateUtils.push(state, route);
+              return {
+                ...StateUtils.push(state, route),
+                isTransitioning: action.immediate !== true,
+              };
             }
           }
         }

src/routers/__tests__/StackRouter-test.js

@@ -692,6 +692,7 @@ describe('StackRouter', () => {
       state
     );
 
+    expect(state2.isTransitioning).toEqual(true);
     expect(state2.index).toEqual(1);
     expect(state2.routes[1].index).toEqual(1);
     expect(state2.routes[1].routes[1].index).toEqual(1);