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_clientserver.vim b/src/testdir/test_clientserver.vim
index a088e17..03d201d 100644
--- a/src/testdir/test_clientserver.vim
+++ b/src/testdir/test_clientserver.vim
@@ -26,6 +26,7 @@
 endfunc
 
 func Test_client_server()
+  let g:test_is_flaky = 1
   let cmd = GetVimCommand()
   if cmd == ''
     throw 'GetVimCommand() failed'