[JS][Integration] Enable custom_metadata integration test

https://github.com/apache/arrow/pull/6556 adds an integration test including custom metadata but JavaScript is skipped.

**Reporter**: [Ben Kietzman](https://issues.apache.org/jira/browse/ARROW-8034) / @bkietz

_{**Note**: *This issue was originally created as [ARROW-8034](https://issues.apache.org/jira/browse/ARROW-8034). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}