Before:
  let g:test_output = [
  \ ['one', 'two', 'thr'],
  \ ['ee', ''],
  \ ['fou'],
  \ [''],
  \ ['r', 'five'],
  \ [],
  \ ['', 'six']
  \]

  let g:expected_result = ['one', 'two', 'three', 'four', 'five', 'six']

After:
  unlet g:test_output
  unlet g:expected_result

Execute (Join the lines):
  let joined_result = []
  for item in g:test_output
    call ale#engine#JoinNeovimOutput(joined_result, item)
  endfor
  AssertEqual g:expected_result, joined_result