Given javascript (Some JavaScript with problems):
  var y = 3+3;
  var y = 3

Before:
  sign unplace *
  let g:actual_sign_list = []
  let g:expected_sign_list = [
  \ ['1', 'ALEWarningSign'],
  \ ['2', 'ALEErrorSign'],
  \]

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

    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(g:actual_sign_list, [l:match[1], l:match[2]])
      endif
    endfor
  endfunction

After:
  sign unplace *
  let g:ale_buffer_info = {}
  delfunction g:CollectSigns
  unlet g:actual_sign_list
  unlet g:expected_sign_list

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

  AssertEqual g:expected_sign_list, g:actual_sign_list