patch 8.2.3477: startup test fails on MS-Windows

Problem:    Startup test fails on MS-Windows.
Solution:   Skip the test if not on Unix.
diff --git a/src/testdir/test_startup.vim b/src/testdir/test_startup.vim
index 897492d..dc32082 100644
--- a/src/testdir/test_startup.vim
+++ b/src/testdir/test_startup.vim
@@ -1297,6 +1297,8 @@
 endfunc
 
 func Test_rename_buffer_on_startup()
+  CheckUnix
+
   let lines =<< trim END
       call writefile(['done'], 'Xresult')
       qa!