Before:
  runtime ale_linters/xml/xmllint.vim

After:
  call ale#linter#Reset()

Execute(The xmllint handler should parse error messages correctly):
  AssertEqual
  \ [
  \   {
  \     'lnum': 1,
  \     'col': 22,
  \     'type': 'W',
  \     'text': 'warning: Unsupported version ''dummy'''
  \   },
  \   {
  \     'lnum': 34,
  \     'col': 1,
  \     'type': 'E',
  \     'text': 'parser error : Start tag expected, ''<'' not found'
  \   }
  \ ],
  \ ale_linters#xml#xmllint#Handle(1, [
  \   'path/to/file.xml:1: warning: Unsupported version ''dummy''',
  \   '<?xml version="dummy"?>',
  \   '                     ^',
  \   '-:34: parser error : Start tag expected, ''<'' not found',
  \   'blahblah>',
  \   '^'
  \ ])