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>

modules/test/utils.go

@@ -5,9 +5,14 @@ package test
 
 import (
 	"net/http"
+	"strings"
 )
 
 // RedirectURL returns the redirect URL of a http response.
 func RedirectURL(resp http.ResponseWriter) string {
 	return resp.Header().Get("Location")
 }
+
+func IsNormalPageCompleted(s string) bool {
+	return strings.Contains(s, `<footer class="page-footer"`) && strings.Contains(s, `</html>`)
+}