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

After:
  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, [{'lnum': 4, 'text': 'foo'}])

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