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_test.go

@@ -0,0 +1,70 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package web
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRouteMock(t *testing.T) {
+	setting.IsInTesting = true
+
+	r := NewRoute()
+	middleware1 := func(resp http.ResponseWriter, req *http.Request) {
+		resp.Header().Set("X-Test-Middleware1", "m1")
+	}
+	middleware2 := func(resp http.ResponseWriter, req *http.Request) {
+		resp.Header().Set("X-Test-Middleware2", "m2")
+	}
+	handler := func(resp http.ResponseWriter, req *http.Request) {
+		resp.Header().Set("X-Test-Handler", "h")
+	}
+	r.Get("/foo", middleware1, RouteMockPoint("mock-point"), middleware2, handler)
+
+	// normal request
+	recorder := httptest.NewRecorder()
+	req, err := http.NewRequest("GET", "http://localhost:8000/foo", nil)
+	assert.NoError(t, err)
+	r.ServeHTTP(recorder, req)
+	assert.Len(t, recorder.Header(), 3)
+	assert.EqualValues(t, "m1", recorder.Header().Get("X-Test-Middleware1"))
+	assert.EqualValues(t, "m2", recorder.Header().Get("X-Test-Middleware2"))
+	assert.EqualValues(t, "h", recorder.Header().Get("X-Test-Handler"))
+	RouteMockReset()
+
+	// mock at "mock-point"
+	RouteMock("mock-point", func(resp http.ResponseWriter, req *http.Request) {
+		resp.Header().Set("X-Test-MockPoint", "a")
+		resp.WriteHeader(http.StatusOK)
+	})
+	recorder = httptest.NewRecorder()
+	req, err = http.NewRequest("GET", "http://localhost:8000/foo", nil)
+	assert.NoError(t, err)
+	r.ServeHTTP(recorder, req)
+	assert.Len(t, recorder.Header(), 2)
+	assert.EqualValues(t, "m1", recorder.Header().Get("X-Test-Middleware1"))
+	assert.EqualValues(t, "a", recorder.Header().Get("X-Test-MockPoint"))
+	RouteMockReset()
+
+	// mock at MockAfterMiddlewares
+	RouteMock(MockAfterMiddlewares, func(resp http.ResponseWriter, req *http.Request) {
+		resp.Header().Set("X-Test-MockPoint", "b")
+		resp.WriteHeader(http.StatusOK)
+	})
+	recorder = httptest.NewRecorder()
+	req, err = http.NewRequest("GET", "http://localhost:8000/foo", nil)
+	assert.NoError(t, err)
+	r.ServeHTTP(recorder, req)
+	assert.Len(t, recorder.Header(), 3)
+	assert.EqualValues(t, "m1", recorder.Header().Get("X-Test-Middleware1"))
+	assert.EqualValues(t, "m2", recorder.Header().Get("X-Test-Middleware2"))
+	assert.EqualValues(t, "b", recorder.Header().Get("X-Test-MockPoint"))
+	RouteMockReset()
+}