Make route middleware/handler mockable (#25766)

Make route middleware/handler mockable (#25766)

To mock a handler:

```go
web.RouteMock(web.MockAfterMiddlewares, func(ctx *context.Context) {
	// ...
})
defer web.RouteMockReset()
```

It helps:

* Test the middleware's behavior (assert the ctx.Data, etc)
* Mock the middleware's behavior (prepare some context data for handler)
* Mock the handler's response for some test cases, especially for some
integration tests and e2e tests.