Build test HTML wrapper pages for the Javascript puzzles.

Build test HTML wrapper pages for the Javascript puzzles.

This should make it less annoying for me to do local testing of the JS
output of a build, before I push a change. There's a new
build.out/jstest directory containing .html files suitable for loading
in a local browser, which refer to the JS files via an appropriate
relative path to the existing build.out/js directory.