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_popup.vim b/src/testdir/test_popup.vim
index 0799df5..2f29163 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -665,6 +665,7 @@
   CheckFeature terminal
   CheckFeature quickfix
   CheckNotGui
+  let g:test_is_flaky = 1
 
   let h = winheight(0)
   if h < 15