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>

tests/integration/admin_config_test.go

@@ -0,0 +1,23 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package integration
+
+import (
+	"net/http"
+	"testing"
+
+	"code.gitea.io/gitea/modules/test"
+	"code.gitea.io/gitea/tests"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAdminConfig(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	session := loginUser(t, "user1")
+	req := NewRequest(t, "GET", "/admin/config")
+	resp := session.MakeRequest(t, req, http.StatusOK)
+	assert.True(t, test.IsNormalPageCompleted(resp.Body.String()))
+}