Before:
    Save g:ale_warn_about_trailing_whitespace

    let g:ale_warn_about_trailing_whitespace = 1

    runtime ale_linters/markdown/pymarkdown.vim

After:
    Restore
    unlet! b:ale_warn_about_trailing_whitespace

    call ale#linter#Reset()

Execute (Should parse error correctly):
  AssertEqual
  \ [
  \   {
        \   'lnum': 1,
        \   'col': 1,
        \   'type': 'M',
        \   'text': 'Headings should be surrounded by blank lines',
        \   'code': 'MD022',
  \   }
  \ ],
  \ ale_linters#markdown#pymarkdown#Handle(bufnr(''), [
  \ 'foo.md:1:1: MD022: Headings should be surrounded by blank lines',
  \ ])

Execute(Warnings about trailing whitespace should be reported by default):
  AssertEqual
  \ [
  \   {
        \   'lnum': 1,
        \   'col': 1,
        \   'type': 'M',
        \   'text': 'who cares',
        \   'code': 'MD009',
  \   },
  \ ],
  \ ale_linters#markdown#pymarkdown#Handle(bufnr(''), [
  \   'foo.md:1:1: MD009: who cares',
  \ ])

Execute(Disabling trailing whitespace warnings should work):
  let b:ale_warn_about_trailing_whitespace = 0

  AssertEqual
  \ [
  \ ],
  \ ale_linters#markdown#pymarkdown#Handle(bufnr(''), [
  \   'foo.md:1:1: MD009: who cares',
  \ ])