Before:
  Save g:ale_nix_nixfmt_executable
  Save g:ale_nix_nixfmt_options

After:
  Restore

Execute(The nixfmt callback should return the correct default values):
  AssertEqual
  \ {
  \   'command': ale#Escape('nixfmt')
  \ },
  \ ale#fixers#nixfmt#Fix(bufnr(''))

Execute(The nixfmt executable and options should be configurable):
  let g:ale_nix_nixfmt_executable = '/path/to/nixfmt'
  let g:ale_nix_nixfmt_options = '--help'

  AssertEqual
  \ {
  \   'command': ale#Escape('/path/to/nixfmt')
  \     . ' --help',
  \ },
  \ ale#fixers#nixfmt#Fix(bufnr(''))