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': 5,
    \   'type': 'E',
    \   'text': 'wat',
    \ },
    \]
  endfunction

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

After:
  delfunction GenerateResults
  call ale#linter#Reset()
  let g:ale_buffer_info = {}
  call clearmatches()

Given testft(A Javscript file with warnings/errors):
  foo
  bar
  baz wat
  line four

Execute(Highlights should be set when a linter runs):
  call ale#Lint()
  call ale#engine#WaitForJobs(2000)

  AssertEqual
  \ [
  \   {'group': 'ALEError', 'id': 4, 'priority': 10, 'pos1': [1, 1, 1]},
  \   {'group': 'ALEWarning', 'id': 5, 'priority': 10, 'pos1': [2, 1, 1]},
  \   {'group': 'ALEError', 'id': 6, 'priority': 10, 'pos1': [3, 5, 1]}
  \ ],
  \ getmatches()

  AssertEqual [4, 5, 6], map(copy(g:ale_buffer_info[bufnr('')].loclist), 'v:val.match_id')

Execute(Existing highlights should be kept):
  call matchaddpos('ALEError', [[1, 2, 1]], 10, 347)
  call matchaddpos('ALEWarning', [[2, 2, 1]], 10, 348)

  call ale#highlight#SetHighlights(bufnr('%'), [
  \ {'bufnr': bufnr('%'), 'match_id': 347, 'type': 'E', 'lnum': 1, 'col': 2},
  \ {'bufnr': bufnr('%'), 'match_id': 348, 'type': 'W', 'lnum': 2, 'col': 2},
  \ {'bufnr': bufnr('%'), 'type': 'E', 'lnum': 3, 'col': 2},
  \ {'bufnr': bufnr('%'), 'type': 'W', 'lnum': 4, 'col': 1},
  \])

  AssertEqual
  \ [
  \   {'group': 'ALEError', 'id': 347, 'priority': 10, 'pos1': [1, 2, 1]},
  \   {'group': 'ALEWarning', 'id': 348, 'priority': 10, 'pos1': [2, 2, 1]},
  \   {'group': 'ALEError', 'id': 7, 'priority': 10, 'pos1': [3, 2, 1]},
  \   {'group': 'ALEWarning', 'id': 8, 'priority': 10, 'pos1': [4, 1, 1]},
  \ ],
  \ getmatches()