patch 8.2.3518: Test_xrestore sometimes fails

Problem:    Test_xrestore sometimes fails.
Solution:   Mark the test as flayky.  Move marking test as flaky to the test
            instead of listing them in runtest.
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index 268c3e9..08ec71b 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
@@ -845,6 +845,7 @@
 endfunc
 
 func Test_diff_screen()
+  let g:test_is_flaky = 1
   CheckScreendump
   CheckFeature menu