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_gui.vim b/src/testdir/test_gui.vim
index c01151c..6b849c7 100644
--- a/src/testdir/test_gui.vim
+++ b/src/testdir/test_gui.vim
@@ -120,6 +120,7 @@
 endfunc
 
 func Test_quoteplus()
+  let g:test_is_flaky = 1
   let skipped = ''
 
   if !g:x11_based_gui