Use `node` for running node.js tests, not `nodejs` (#25651)

This should solve the problem of getting `nodejs: not found` when
running `paver test_js_run -s lms` in lms-shell, which cropped up in
the past few days.

I don't know why this stopped working recently, but I don't have `nodejs`
on my recently re-created devstack, and some people with an older devstack
have both. Maybe a recent node.js release got rid of an alias.