Before:
  call ale#assert#SetUpFixerTest('markdown', 'markdownlint')

After:
  call ale#assert#TearDownFixerTest()

Execute:
  AssertFixer {
  \ 'command': ale#Escape('markdownlint') . ' --fix',
  \}

Execute:
  let g:ale_markdownlint_executable = 'custom_markdownlint'
  
  AssertFixer {
  \ 'command': ale#Escape('custom_markdownlint') . ' --fix',
  \}