Before:
  Save g:ale_buffer_info
  Save &filetype

  " Set up items in other buffers which should set in this one.
  let g:ale_buffer_info = {}
  call ale#engine#InitBufferInfo(bufnr('') + 1)
  let g:ale_buffer_info[bufnr('') + 1].loclist =
  \ ale#engine#FixLocList(bufnr('') + 1, 'linter_one', [
  \   {'lnum': 1, 'filename': expand('%:p'), 'text': 'foo'},
  \   {'lnum': 2, 'filename': expand('%:p'), 'text': 'bar'},
  \   {'lnum': 2, 'text': 'ignore this one'},
  \ ])
  call ale#engine#InitBufferInfo(bufnr('') + 2)
  let g:ale_buffer_info[bufnr('') + 2].loclist =
  \ ale#engine#FixLocList(bufnr('') + 2, 'linter_two', [
  \   {'lnum': 1, 'filename': expand('%:p'), 'text': 'foo'},
  \   {'lnum': 3, 'filename': expand('%:p'), 'text': 'baz'},
  \   {'lnum': 5, 'text': 'ignore this one'},
  \ ])

  call ale#linter#Define('foobar', {
  \ 'name': 'linter_one',
  \ 'callback': 'WhoCares',
  \ 'executable': 'echo',
  \ 'command': 'echo',
  \ 'lint_file': 1,
  \})
  call ale#linter#Define('foobar', {
  \ 'name': 'linter_two',
  \ 'callback': 'WhoCares',
  \ 'executable': 'echo',
  \ 'command': 'echo',
  \})

After:
  call ale#engine#Cleanup(bufnr(''))
  Restore
  call ale#linter#Reset()

  " Items and markers, etc.
  call setloclist(0, [])
  call clearmatches()
  sign unplace *

Given foobar(A file with some lines):
  foo
  bar
  baz

Execute(Problems found from previously opened buffers should be set when linting for the first time):
  call ale#engine#RunLinters(bufnr(''), ale#linter#Get(&filetype), 0)

  AssertEqual
  \ [
  \   {'lnum': 1, 'bufnr': bufnr(''), 'col': 0, 'valid': 1, 'vcol': 0, 'nr': -1, 'type': 'E', 'pattern': '', 'text': 'foo'},
  \   {'lnum': 2, 'bufnr': bufnr(''), 'col': 0, 'valid': 1, 'vcol': 0, 'nr': -1, 'type': 'E', 'pattern': '', 'text': 'bar'},
  \   {'lnum': 3, 'bufnr': bufnr(''), 'col': 0, 'valid': 1, 'vcol': 0, 'nr': -1, 'type': 'E', 'pattern': '', 'text': 'baz'},
  \ ],
  \ getloclist(0)