CppunitTest_desktop_lib: fix stack-use-after-return

The LOK callback objects registered are on-stack objects, so we must be
sure that references no longer used after we return.

Process idle events, so that desktop::CallbackHandler::flush() won't try
to use the reference after return.

Change-Id: I65d324cb9cef4fd1776d0f610d8d096717e0e833
Reviewed-on: https://gerrit.libreoffice.org/30034Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>

desktop/qa/desktop_lib/test_desktop_lib.cxx

@@ -1639,6 +1639,7 @@ void DesktopLOKTest::testPaintPartTile()
     // the first view, so there were no invalidations.
     CPPUNIT_ASSERT(aView1.m_bTilesInvalidated);
 
+    Scheduler::ProcessEventsToIdle();
     mxComponent->dispose();
     mxComponent.clear();
     comphelper::LibreOfficeKit::setActive(false);
@@ -1678,6 +1679,7 @@ void DesktopLOKTest::testWriterCommentInsertCursor()
     // inserted the comment.
     CPPUNIT_ASSERT(aView1.m_aOwnCursor.IsEmpty());
 
+    Scheduler::ProcessEventsToIdle();
     mxComponent->dispose();
     mxComponent.clear();
     comphelper::LibreOfficeKit::setActive(false);