fix: ruff not registered as fixer (#4393)

this commit fixes the issue reported at:
https://github.com/dense-analysis/ale/issues/4301#issuecomment-1359289391

it also had some code refactored and
tests added for ruff as fixer (missed in PR #4347).

autoload/ale/fix/registry.vim

@@ -580,6 +580,11 @@ let s:default_registry = {
 \       'function': 'ale#fixers#raco_fmt#Fix',
 \       'suggested_filetypes': ['racket'],
 \       'description': 'Fix Racket files with raco fmt.',
+\   },
+\   'ruff': {
+\       'function': 'ale#fixers#ruff#Fix',
+\       'suggested_filetypes': ['python'],
+\       'description': 'Fix python files with ruff.',
 \   }
 \}