Before:
  runtime! ale_linters/markdown/redpen.vim

After:
  call ale#linter#Reset()

Execute(redpen handler should handle errors output):
  AssertEqual
  \ [
  \   {
  \     'lnum': 1,
  \     'col': 9,
  \     'end_lnum': 1,
  \     'end_col': 15,
  \     'text': 'Found possibly misspelled word "plugin". (Spelling)',
  \     'type': 'W',
  \   },
  \   {
  \     'lnum': 1,
  \     'col': 1,
  \     'text': 'Found possibly misspelled word "NeoVim". (Spelling)',
  \     'type': 'W',
  \   },
  \ ],
  \ ale_linters#markdown#redpen#HandleErrors(bufnr(''), [
  \   '[',
  \   '  {',
  \   '    "document": "test.md",',
  \   '    "errors": [',
  \   '      {',
  \   '        "sentence": "ALE is a plugin for providing linting in NeoVim and Vim 8 while you edit your text files.",',
  \   '        "endPosition": {',
  \   '          "offset": 15,',
  \   '          "lineNum": 1',
  \   '        },',
  \   '        "validator": "Spelling",',
  \   '        "lineNum": 1,',
  \   '        "sentenceStartColumnNum": 0,',
  \   '        "message": "Found possibly misspelled word \"plugin\".",',
  \   '        "startPosition": {',
  \   '          "offset": 9,',
  \   '          "lineNum": 1',
  \   '        }',
  \   '      },',
  \   '      {',
  \   '        "sentence": "ALE is a plugin for providing linting in NeoVim and Vim 8 while you edit your text files.",',
  \   '        "validator": "Spelling",',
  \   '        "lineNum": 1,',
  \   '        "sentenceStartColumnNum": 0,',
  \   '        "message": "Found possibly misspelled word \"NeoVim\"."',
  \   '      }',
  \   '    ]',
  \   '  }',
  \   ']',
  \ ])

Execute(redpen handler should no error output):
  AssertEqual
  \ [],
  \ ale_linters#markdown#redpen#HandleErrors(bufnr(''), [
  \   '[',
  \   '  {',
  \   '    "document": "test.md",',
  \   '    "errors": []',
  \   '  }',
  \   ']',
  \ ])