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.

src/testdir/test_paste.vim

@@ -136,6 +136,8 @@ endfunc
 
 func Test_xrestore()
   CheckFeature xterm_clipboard
+  let g:test_is_flaky = 1
+
   let display = $DISPLAY
   new
   call CheckCopyPaste()