Use GetDisplayName() instead of DisplayName() to generate rss feeds (followup)

The test only exists in Forgejo and the behavior it verifies now require
setting.UI.DefaultShowFullName to be true.

tests/integration/api_feed_plain_text_titles_test.go

@@ -7,6 +7,8 @@ import (
 	"net/http"
 	"testing"
 
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/test"
 	"code.gitea.io/gitea/tests"
 
 	"github.com/stretchr/testify/assert"
@@ -15,6 +17,7 @@ import (
 func TestFeedPlainTextTitles(t *testing.T) {
 	// This test verifies that items' titles in feeds are generated as plain text.
 	// See https://codeberg.org/forgejo/forgejo/pulls/1595
+	defer test.MockVariableValue(&setting.UI.DefaultShowFullName, true)()
 
 	t.Run("Feed plain text titles", func(t *testing.T) {
 		t.Run("Atom", func(t *testing.T) {