Before:
  Save g:ale_buffer_info

  let g:ale_buffer_info = {
  \ bufnr(''): {
  \   'loclist': [{'bufnr': bufnr(''), 'lnum': 4, 'col': 1, 'text': 'foo'}]
  \ },
  \}

  let g:original_buffer = bufnr('%')
  noautocmd new

After:
  Restore

  unlet! g:original_buffer

Execute(Errors should be set in the loclist for the original buffer, not the new one):
  call ale#list#SetLists(
  \ g:original_buffer,
  \ g:ale_buffer_info[(g:original_buffer)].loclist,
  \ )

  AssertEqual [], ale#test#GetLoclistWithoutNewerKeys()
  AssertEqual 1, len(getloclist(bufwinid(g:original_buffer)))
  AssertEqual 'foo', getloclist(bufwinid(g:original_buffer))[0].text