[VirtualFileSystem] Add unit test that showcases YAMLVFSWriter bug

[VirtualFileSystem] Add unit test that showcases YAMLVFSWriter bug

This scenario generates a broken YAML mapping as illustrated below.

 {
   'type': 'directory',
   'name': "c",
   'contents': [
     {
       'type': 'file',
       'name': "d",
       'external-contents': "//root/a/c/d"
     }            {
       'type': 'file',
       'name': "e",
       'external-contents': "//root/a/c/e"
     }            {
       'type': 'file',
       'name': "f",
       'external-contents': "//root/a/c/f"
     }
   ]
 },