[SCHEMA] Very minor tune ups to the test_rule_objects test while tryi…

…ng to grasp its function (#1728)

* RF: minor simplification of Python code

If the point is to go through values, just go through .values()
without iterating through keys() and then dereferencing via [key]

* Raise AssertionError in the test, not ValueError

Since then also it is not clear if that is indeed some ValueError from within code.
I also added a string which summarizes the problem.

* Update tools/schemacode/bidsschematools/data/tests/test_rules.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: Chris Markiewicz <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tools/schemacode/bidsschematools/data/tests/test_rules.py

@@ -83,8 +83,7 @@ def test_rule_objects(schema_obj):
                 if object_type in ["extensions", "suffixes"]:
                     # Some object types are referenced via their "value" fields in the rules
                     object_values = [
-                        schema_obj["objects"][object_type][k]["value"]
-                        for k in schema_obj["objects"][object_type].keys()
+                        value["value"] for value in schema_obj["objects"][object_type].values()
                     ]
                 else:
                     # But other object types are referenced via their keys
@@ -100,7 +99,7 @@ def test_rule_objects(schema_obj):
 
     if not_found:
         not_found_string = "\n".join([f"{'.'.join(path)} == {val}" for path, val in not_found])
-        raise ValueError(not_found_string)
+        raise AssertionError(f"Undefined objects found in rules: {not_found_string}")
 
 
 @pytest.mark.validate_schema