[JS] Run build before tests

The tests exercise the compiled files but the test scripts do not automatically run the builds.

**Reporter**: [Dominik Moritz](https://issues.apache.org/jira/browse/ARROW-12276) / @domoritz

_{**Note**: *This issue was originally created as [ARROW-12276](https://issues.apache.org/jira/browse/ARROW-12276). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}