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_diffmode.vim

@@ -845,6 +845,7 @@ func VerifyInternal(buf, dumpfile, extra)
 endfunc
 
 func Test_diff_screen()
+  let g:test_is_flaky = 1
   CheckScreendump
   CheckFeature menu