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.

modules/web/routemock.go

@@ -0,0 +1,61 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package web
+
+import (
+	"net/http"
+
+	"code.gitea.io/gitea/modules/setting"
+)
+
+// MockAfterMiddlewares is a general mock point, it's between middlewares and the handler
+const MockAfterMiddlewares = "MockAfterMiddlewares"
+
+var routeMockPoints = map[string]func(next http.Handler) http.Handler{}
+
+// RouteMockPoint registers a mock point as a middleware for testing, example:
+//
+//	r.Use(web.RouteMockPoint("my-mock-point-1"))
+//	r.Get("/foo", middleware2, web.RouteMockPoint("my-mock-point-2"), middleware2, handler)
+//
+// Then use web.RouteMock to mock the route execution.
+// It only takes effect in testing mode (setting.IsInTesting == true).
+func RouteMockPoint(pointName string) func(next http.Handler) http.Handler {
+	if !setting.IsInTesting {
+		return nil
+	}
+	routeMockPoints[pointName] = nil
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if h := routeMockPoints[pointName]; h != nil {
+				h(next).ServeHTTP(w, r)
+			} else {
+				next.ServeHTTP(w, r)
+			}
+		})
+	}
+}
+
+// RouteMock uses the registered mock point to mock the route execution, example:
+//
+//	defer web.RouteMockReset()
+//	web.RouteMock(web.MockAfterMiddlewares, func(ctx *context.Context) {
+//		ctx.WriteResponse(...)
+//	}
+//
+// Then the mock function will be executed as a middleware at the mock point.
+// It only takes effect in testing mode (setting.IsInTesting == true).
+func RouteMock(pointName string, h any) {
+	if _, ok := routeMockPoints[pointName]; !ok {
+		panic("route mock point not found: " + pointName)
+	}
+	routeMockPoints[pointName] = toHandlerProvider(h)
+}
+
+// RouteMockReset resets all mock points (no mock anymore)
+func RouteMockReset() {
+	for k := range routeMockPoints {
+		routeMockPoints[k] = nil // keep the keys because RouteMock will check the keys to make sure no misspelling
+	}
+}