Execute(Prefix filtering should work for Lists of strings):
  AssertEqual
  \ ['FooBar', 'foo'],
  \ ale#completion#Filter(['FooBar', 'FongBar', 'baz', 'foo'], 'foo')
  AssertEqual
  \ ['FooBar', 'FongBar', 'baz', 'foo'],
  \ ale#completion#Filter(['FooBar', 'FongBar', 'baz', 'foo'], '.')

Execute(Prefix filtering should work for completion items):
  AssertEqual
  \ [{'word': 'FooBar'}, {'word': 'foo'}],
  \ ale#completion#Filter(
  \   [
  \     {'word': 'FooBar'},
  \     {'word': 'FongBar'},
  \     {'word': 'baz'},
  \     {'word': 'foo'},
  \   ],
  \   'foo'
  \ )
  AssertEqual
  \ [
  \   {'word': 'FooBar'},
  \   {'word': 'FongBar'},
  \   {'word': 'baz'},
  \   {'word': 'foo'},
  \ ],
  \ ale#completion#Filter(
  \   [
  \     {'word': 'FooBar'},
  \     {'word': 'FongBar'},
  \     {'word': 'baz'},
  \     {'word': 'foo'},
  \   ],
  \   '.'
  \ )