Workaround: Skip Test_writedelay on GUI test

Since Vim v8.2.2837, Test_VIM_POSIX is added, Test_writedelay fails
sometimes.  Because, 'writedelay' makes no effect while its GUI window
focus is lost, and Test_VIM_POSIX is executed prior to Test_writedelay
and opens the own new GUI window so the main window loses focus once.

Author: ichizok

src/testdir/test_options.vim

@@ -779,6 +779,11 @@ func Test_local_scrolloff()
 endfunc
 
 func Test_writedelay()
+  " Workaround for MacVim GUI: This test fails sometimes after Test_VIM_POSIX.
+  " Because, 'writedelay' makes no effect while its GUI window focus is lost,
+  " and Test_VIM_POSIX opens the own new GUI window so the main window loses
+  " focus once.
+  CheckNotGui
   CheckFunction reltimefloat
 
   new