patch 8.1.0520: screen diff test sometimes fails

Problem:    Screen diff test sometimes fails.
Solution:   Add to list of flaky tests.
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index ff32abc..cacfa75 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -279,6 +279,7 @@
       \ 'Test_collapse_buffers()',
       \ 'Test_communicate()',
       \ 'Test_cwd()',
+      \ 'Test_diff_screen()',
       \ 'Test_exit_callback_interval()',
       \ 'Test_nb_basic()',
       \ 'Test_oneshot()',