[BUG] Render correct label link

- Render the correct label for pull requests, it should link to the pull
requests list and not the issue list.
- Add unit test.
- Resolves https://codeberg.org/forgejo/forgejo/issues/3183

(cherry picked from commit 7a97c05206)

modules/templates/main_test.go

@@ -0,0 +1,24 @@
+// Copyright 2024 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package templates_test
+
+import (
+	"context"
+	"testing"
+
+	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/markup"
+
+	_ "code.gitea.io/gitea/models"
+	_ "code.gitea.io/gitea/models/issues"
+)
+
+func TestMain(m *testing.M) {
+	markup.Init(&markup.ProcessorHelper{
+		IsUsernameMentionable: func(ctx context.Context, username string) bool {
+			return username == "mention-user"
+		},
+	})
+	unittest.MainTest(m)
+}