Before:
  Save g:ale_nix_nixpkgsfmt_executable
  Save g:ale_nix_nixpkgsfmt_options

After:
  Restore

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

Execute(The nixpkgs-fmt executable and options should be configurable):
  let g:ale_nix_nixpkgsfmt_executable = '/path/to/nixpkgs-fmt'
  let g:ale_nix_nixpkgsfmt_options = '-h'

  AssertEqual
  \ {
  \   'command': ale#Escape('/path/to/nixpkgs-fmt')
  \     . ' -h',
  \ },
  \ ale#fixers#nixpkgsfmt#Fix(bufnr(''))