Before:
  function! GenerateResults(buffer, output)
    return [
    \ {
    \   'lnum': 1,
    \   'col': 1,
    \   'type': 'E',
    \   'text': 'foo',
    \ },
    \ {
    \   'lnum': 2,
    \   'col': 1,
    \   'type': 'W',
    \   'text': 'bar',
    \ },
    \ {
    \   'lnum': 3,
    \   'col': 1,
    \   'type': 'E',
    \   'text': 'baz',
    \ },
    \ {
    \   'lnum': 4,
    \   'col': 1,
    \   'type': 'E',
    \   'text': 'use this one',
    \ },
    \ {
    \   'lnum': 4,
    \   'col': 2,
    \   'type': 'W',
    \   'text': 'ignore this one',
    \ },
    \ {
    \   'lnum': 5,
    \   'col': 1,
    \   'type': 'W',
    \   'text': 'ignore this one',
    \ },
    \ {
    \   'lnum': 5,
    \   'col': 2,
    \   'type': 'E',
    \   'text': 'use this one',
    \ },
    \]
  endfunction

  function! ParseSigns()
    redir => l:output
      silent sign place
    redir END

    return map(
    \ split(l:output, '\n')[2:],
    \ 'matchlist(v:val, ''^.*=\(\d\+\).*=\(\d\+\).*=\(.*\)$'')[1:3]',
    \)
  endfunction

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

After:
  unlet! g:loclist
  delfunction GenerateResults
  delfunction ParseSigns
  call ale#linter#Reset()
  sign unplace *

Given testft(A file with warnings/errors):
  foo
  bar
  baz
  fourth line
  fifth line

Execute(The current signs should be set for running a job):
  call ale#Lint()
  call ale#engine#WaitForJobs(2000)

  AssertEqual
  \ [
  \   ['1', '1000001', 'ALEErrorSign'],
  \   ['2', '1000002', 'ALEWarningSign'],
  \   ['3', '1000003', 'ALEErrorSign'],
  \   ['4', '1000004', 'ALEErrorSign'],
  \   ['5', '1000005', 'ALEErrorSign'],
  \ ],
  \ ParseSigns()


Execute(Loclist items with sign_id values should be kept):
  exec 'sign place 1000347 line=15 name=ALEErrorSign buffer=' . bufnr('%')
  exec 'sign place 1000348 line=16 name=ALEWarningSign buffer=' . bufnr('%')

  let g:loclist = [
  \ {'lnum': 1, 'col': 1, 'type': 'E', 'text': 'a', 'sign_id': 1000347},
  \ {'lnum': 2, 'col': 1, 'type': 'W', 'text': 'b', 'sign_id': 1000348},
  \ {'lnum': 3, 'col': 1, 'type': 'E', 'text': 'c'},
  \ {'lnum': 4, 'col': 1, 'type': 'W', 'text': 'd'},
  \ {'lnum': 15, 'col': 2, 'type': 'W', 'text': 'e'},
  \ {'lnum': 16, 'col': 2, 'type': 'E', 'text': 'f'},
  \]

  call ale#sign#SetSigns(bufnr('%'), g:loclist)

  " Line numbers should be changed, sign_id values should be replaced,
  " and items should be sorted again.
  AssertEqual
  \ [
  \   {'lnum': 3, 'col': 1, 'type': 'E', 'text': 'c', 'sign_id': 1000001},
  \   {'lnum': 4, 'col': 1, 'type': 'W', 'text': 'd', 'sign_id': 1000002},
  \   {'lnum': 15, 'col': 1, 'type': 'E', 'text': 'a', 'sign_id': 1000003},
  \   {'lnum': 15, 'col': 2, 'type': 'W', 'text': 'e', 'sign_id': 1000003},
  \   {'lnum': 16, 'col': 1, 'type': 'W', 'text': 'b', 'sign_id': 1000004},
  \   {'lnum': 16, 'col': 2, 'type': 'E', 'text': 'f', 'sign_id': 1000004},
  \ ],
  \ g:loclist

  " Items should be grouped again. We should see error signs, where there
  " were warnings before, and errors where there were errors and where we
  " now have new warnings.
  AssertEqual
  \ [
  \   ['3', '1000001', 'ALEErrorSign'],
  \   ['4', '1000002', 'ALEWarningSign'],
  \   ['15', '1000003', 'ALEErrorSign'],
  \   ['16', '1000004', 'ALEErrorSign'],
  \ ],
  \ ParseSigns()