Before:
  function! GenerateResults(buffer, output)
    return [
    \ {
    \   'lnum': 1,
    \   'col': 1,
    \   'bufnr': bufnr('%'),
    \   'vcol': 0,
    \   'nr': -1,
    \   'type': 'E',
    \   'text': 'foo',
    \ },
    \ {
    \   'lnum': 2,
    \   'col': 1,
    \   'bufnr': bufnr('%'),
    \   'vcol': 0,
    \   'nr': -1,
    \   'type': 'W',
    \   'text': 'bar',
    \ },
    \ {
    \   'lnum': 3,
    \   'col': 1,
    \   'bufnr': bufnr('%'),
    \   'vcol': 0,
    \   'nr': -1,
    \   'type': 'E',
    \   'text': 'baz',
    \ },
    \]
  endfunction

  call ale#linter#Define('testft', {
  \ 'name': 'x',
  \ 'executable': 'echo',
  \ 'command': 'echo',
  \ 'callback': 'GenerateResults',
  \})

After:
  call ale#linter#Reset()
  delfunction GenerateResults
  unlet! g:output

Given testft(A Javscript file with warnings/errors):
  foo
  bar
  baz

Execute:
  call ale#Lint()
  call ale#engine#WaitForJobs(2000)

  redir => g:output
    :sign place
  redir END

  AssertEqual
  \ [
  \   ['1', '1000001', 'ALEErrorSign'],
  \   ['2', '1000002', 'ALEWarningSign'],
  \   ['3', '1000003', 'ALEErrorSign'],
  \ ],
  \ map(
  \   split(g:output, '\n')[2:],
  \   'matchlist(v:val, "[^=]*=\\(\\d\\+\\)[^=]*=\\(\\d\\+\\).*\\(ALE.*\\)$")[1:3]'
  \ )