patch 8.2.4349: FileChangedShell test fails on MS-Windows

Problem:    FileChangedShell test fails on MS-Windows.
Solution:   Skip the test on MS-Windows.
diff --git a/src/testdir/test_filechanged.vim b/src/testdir/test_filechanged.vim
index 0b5b8e0..ee9e80b 100644
--- a/src/testdir/test_filechanged.vim
+++ b/src/testdir/test_filechanged.vim
@@ -139,6 +139,8 @@
 
 func Test_FileChangedShell_edit_dialog()
   CheckNotGui
+  " FIXME: why does this not work on MS-Windows?
+  CheckUnix
 
   new Xchanged_r
   call setline(1, 'reload this')