vim-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.
4e03933726

src/nvim/testdir/runtest.vim

@@ -289,6 +289,7 @@ let s:flaky_tests = [
       \ 'Test_oneshot()',
       \ 'Test_out_cb()',
       \ 'Test_paused()',
+      \ 'Test_popup_and_window_resize()',
       \ 'Test_quoteplus()',
       \ 'Test_quotestar()',
       \ 'Test_reltime()',