Test backup exists

Author: WardBrian

test/data/.gitignore

@@ -6,4 +6,5 @@
 # and we re-ignore hpp and exe files
 *.hpp
 *.exe
-!return_one.hpp
+*.*bak
+!return_one.hpp

test/test_model.py

@@ -406,6 +406,13 @@ def test_model_format_deprecations(self):
         self.assertNotIn("<-", formatted)
         self.assertEqual(formatted.count('('), 0)
 
+        shutil.copy(stan, stan + '.testbak')
+        try:
+            model.format(overwrite_file=True, canonicalize=True)
+            self.assertTrue(os.path.exists(stan + '.bak'))
+        finally:
+            shutil.copy(stan + '.testbak', stan)
+
     @pytest.mark.skipif(
         cmdstan_version_before(2, 29), reason='Options only available later'
     )