Before:
  call ale#fix#registry#Clear()
  call ale#test#SetFilename('test.js')
  call ale#fix#registry#Add('prettier', '', ['javascript'], 'prettier')
  call ale#fix#registry#Add('eslint', '', ['javascript'], 'eslint')
  setfiletype javascript

Execute(completeFixers returns all of the applicable fixers without an arglead):
  AssertEqual ['eslint', 'prettier'],
  \ ale#fix#registry#CompleteFixers('', 'ALEFix ', 7)

Execute(completeFixers returns all of the applicable fixers without an arglead):
  AssertEqual ['prettier'],
  \ ale#fix#registry#CompleteFixers('pre', 'ALEFix ', 10)