One last attempt at fixing UITest_writer_tests for Jenkins ASan+UBSan build

...where <https://ci.libreoffice.org/job/lo_ubsan/997/> still triggered the
assert at sw/source/core/layout/findfrm.cxx:1702 during the second .uno:Undo.
Lets see if a little sleep "fixes" that.

Change-Id: I659ea86aab03f2d39913122af2cc8869abff5578
Reviewed-on: https://gerrit.libreoffice.org/58777
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

sw/qa/uitest/writer_tests/tdf79569.py

@@ -60,6 +60,7 @@ class tdf79569(UITestCase):
         print("DEBUG.18", file=stderr);
         toolkit_ex.processEventsToIdle()
         print("DEBUG.19", file=stderr);
+        sleep(3)
         self.xUITest.executeCommand(".uno:Undo")
         print("DEBUG.20", file=stderr);
         toolkit_ex.processEventsToIdle()