Before:
  Save g:ale_warn_about_trailing_whitespace

  let g:ale_warn_about_trailing_whitespace = 1

  runtime! ale/handlers/yamllint.vim

After:
  Restore

  unlet! b:ale_warn_about_trailing_whitespace

  call ale#linter#Reset()

Execute(Problems should be parsed correctly for yamllint):
  AssertEqual
  \ [
  \   {
  \     'lnum': 1,
  \     'col': 1,
  \     'type': 'W',
  \     'text': 'missing document start "---"',
  \     'code': 'document-start',
  \   },
  \   {
  \     'lnum': 2,
  \     'col': 1,
  \     'type': 'E',
  \     'text': 'syntax error: expected the node content, but found ''<stream end>''',
  \   },
  \ ],
  \ ale#handlers#yamllint#Handle(bufnr(''), [
  \   'something.yaml:1:1: [warning] missing document start "---" (document-start)',
  \   'something.yml:2:1: [error] syntax error: expected the node content, but found ''<stream end>''',
  \ ])

Execute(The yamllint handler should respect ale_warn_about_trailing_whitespace):
  AssertEqual
  \ [
  \   {
  \     'lnum': 5,
  \     'col': 18,
  \     'type': 'E',
  \     'text': 'trailing spaces',
  \     'code': 'trailing-spaces',
  \   },
  \ ],
  \ ale#handlers#yamllint#Handle(bufnr(''), [
  \   'something.yml:5:18: [error] trailing spaces (trailing-spaces)',
  \ ])

  let b:ale_warn_about_trailing_whitespace = 0

  AssertEqual
  \ [
  \ ],
  \ ale#handlers#yamllint#Handle(bufnr(''), [
  \   'something.yml:5:18: [error] trailing spaces (trailing-spaces)',
  \ ])