Before:
  Save g:ale_run_synchronously

  let b:old_filetype = &filetype
  let g:ale_run_synchronously = 1

  noautocmd let &filetype = 'foobar'

  function! TestCallback(buffer, output)
    return [{'text': 'x', 'lnum': 1}]
  endfunction

  call ale#linter#Define('foobar', {
  \ 'name': 'buffer_linter',
  \ 'callback': 'TestCallback',
  \ 'executable': has('win32') ? 'cmd': 'true',
  \ 'command': 'true',
  \ 'read_buffer': 0,
  \})

  call ale#linter#Define('foobar2', {
  \ 'name': 'buffer_linter',
  \ 'callback': 'TestCallback',
  \ 'executable': has('win32') ? 'cmd': 'true',
  \ 'command': 'true',
  \ 'read_buffer': 0,
  \})

After:
  Restore

  noautocmd let &filetype = b:old_filetype
  unlet b:old_filetype
  delfunction TestCallback

  if has_key(g:ale_buffer_info, bufnr(''))
    call remove(g:ale_buffer_info, bufnr(''))
  endif

  call ale#Queue(0)

Execute(Error should be removed when the filetype changes to something else we cannot check):
  call ale#Queue(0)
  sleep 1ms

  AssertEqual 1, len(getloclist(0))

  noautocmd let &filetype = 'foobar2'

  call ale#Queue(0)
  sleep 1ms

  " We should get some items from the second filetype.
  AssertEqual 1, len(getloclist(0))

  noautocmd let &filetype = 'xxx'

  call ale#Queue(0)
  sleep 1ms

  AssertEqual 0, len(getloclist(0))