Add test for completion count in cmdutils.register

tests/unit/commands/test_cmdutils.py

@@ -173,6 +173,13 @@ class TestRegister:
             pass
         assert cmdutils.cmd_dict['fun'].hide
 
+    def test_wrong_completion_count(self):
+        with pytest.raises(ValueError):
+            @cmdutils.register(completion=['one', 'two'])
+            def fun(arg):
+                """Blah."""
+                pass
+
     def test_star_args(self):
         """Check handling of *args"""
         @cmdutils.register()