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_quotestar.vim b/src/testdir/test_quotestar.vim
index 6b8aebf..b669019 100644
--- a/src/testdir/test_quotestar.vim
+++ b/src/testdir/test_quotestar.vim
@@ -131,6 +131,7 @@
 endfunc
 
 func Test_quotestar()
+  let g:test_is_flaky = 1
   let skipped = ''
 
   let quotestar_saved = @*