Data-encoding: remove some JS tests, fix alias

What

This MR:

• removes JS tests of data-encoding that require --stack-size;
• renames the alias for JS tests to runtest_js.

Why

The version of node.js used by the CI doesn't know about --stack-size.

runtest_js is needed so that once we plug the tests in the CI, the CI knows to run them in the job that has node.js.

How

With my keyboard.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR