[VirtualFileSystem] Add unit test that showcases another YAMLVFSWriter bug

This scenario generates another broken YAML mapping as illustrated below.

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