Execute (syntax/qf.vim handles unused customqf):
  if exists('*neomake#quickfix#FormatQuickfix')
    delfunction neomake#quickfix#FormatQuickfix
    delfunction neomake#quickfix#is_enabled
  endif

  call setloclist(0, [{
      \ 'lnum': 1,
      \ 'text': 'E123 error nmcfg:{"name": "python", "short": "py"}',
      \}])
  lopen
  lclose

  " Does not autoload files.
  Assert !exists('*neomake#quickfix#is_enabled')