Fix admin config page error, use tests to cover the admin config and 500 error page (#24965)

The admin config page has been broken for many many times, a little
refactoring would make this page panic.

So, add a test for it, and add another test to cover the 500 error page.

Co-authored-by: Giteabot <teabot@gitea.io>

routers/common/errpage_test.go

@@ -0,0 +1,41 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package common
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"path/filepath"
+	"testing"
+
+	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/test"
+	"code.gitea.io/gitea/modules/web/middleware"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRenderPanicErrorPage(t *testing.T) {
+	w := httptest.NewRecorder()
+	req := &http.Request{URL: &url.URL{}}
+	req = req.WithContext(middleware.WithContextData(context.Background()))
+	RenderPanicErrorPage(w, req, errors.New("fake panic error (for test only)"))
+	respContent := w.Body.String()
+	assert.Contains(t, respContent, `class="page-content status-page-500"`)
+	assert.Contains(t, respContent, `</html>`)
+
+	// the 500 page doesn't have normal pages footer, it makes it easier to distinguish a normal page and a failed page.
+	// especially when a sub-template causes page error, the HTTP response code is still 200,
+	// the different "footer" is the only way to know whether a page is fully rendered without error.
+	assert.False(t, test.IsNormalPageCompleted(respContent))
+}
+
+func TestMain(m *testing.M) {
+	unittest.MainTest(m, &unittest.TestOptions{
+		GiteaRootPath: filepath.Join("..", ".."),
+	})
+}