UITest_writer_tests5: disable flaky print preview test

Change-Id: I22431e026c1802e33064aa60267de0edbdf856ca
Reviewed-on: https://gerrit.libreoffice.org/71253
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

sw/qa/uitest/writer_tests5/tdf121591.py

@@ -14,6 +14,8 @@ from uitest.uihelper.common import select_pos
 class tdf121591(UITestCase):
 
    def test_tdf121591_frame_print_preview(self):
+        # FIXME unstable test
+        return
         self.ui_test.create_doc_in_start_center("writer")
         document = self.ui_test.get_component()
         xWriterDoc = self.xUITest.getTopFocusWindow()