Before:
    let g:buffer = bufnr('%')

    let g:ale_buffer_info = {
    \  g:buffer : {},
    \  10347: {},
    \}

After:
    unlet! g:buffer
    let g:ale_buffer_info = {}

Execute('ALE globals should be cleared when the buffer is closed.'):
    :q!
    AssertEqual {10347: {}}, g:ale_buffer_info