patch 8.1.2162: popup resize test is flaky

Problem:    Popup resize test is flaky. (Christian Brabandt)
Solution:   Add the function to the list of flaky tests.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index a8eb5e2..e22557d 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -340,6 +340,7 @@
       \ 'Test_quotestar()',
       \ 'Test_raw_one_time_callback()',
       \ 'Test_reltime()',
+      \ 'Test_popup_and_window_resize()',
       \ 'Test_server_crash()',
       \ 'Test_state()',
       \ 'Test_term_mouse_double_click_to_create_tab()',