Given foobar (Some imaginary filetype):
  var y = 3+3;
  var y = 3

Before:
  sign unplace *

  function! TestCallback(buffer, output)
    return [
    \ {'lnum': 1, 'text': 'foo', 'type': 'W'},
    \ {'lnum': 2, 'text': 'foo', 'type': 'E'},
    \]
  endfunction

  function! CollectSigns()
    redir => l:output
       silent exec 'sign place'
    redir END

    let l:actual_sign_list = []

    for l:line in split(l:output, "\n")
      let l:match = matchlist(l:line, 'line=\(\d\+\).*name=\(ALE[a-zA-Z]\+\)')

      if len(l:match) > 0
        call add(l:actual_sign_list, [l:match[1], l:match[2]])
      endif
    endfor

    return l:actual_sign_list
  endfunction

  call ale#linter#Define('foobar', {
  \ 'name': 'testlinter',
  \ 'callback': 'TestCallback',
  \ 'executable': has('win32') ? 'cmd' : 'echo',
  \ 'command': has('win32') ? 'echo foo bar' : '/bin/sh -c ''echo foo bar''',
  \})


After:
  delfunction TestCallback
  delfunction CollectSigns

  sign unplace *
  let g:ale_buffer_info = {}
  call ale#linter#Reset()

Execute(The signs should be updated after linting is done):
  call ale#Lint()
  call ale#engine#WaitForJobs(2000)

  AssertEqual [['1', 'ALEWarningSign'], ['2', 'ALEErrorSign']], CollectSigns()