cli: fix broken 'test' command

Broken by 92b6753b.

The test command of the CLI didn't work because Javascript cannot work
with a Java String[] array as if it is a JS array, I was surprised to see
92b6753b and I assumed it works but
apparently the test command got broken.

This patch properly returning a Scriptable object that Javascript
code understands, it acquires a thread specific Rhino Context that
is used to cast the test names to a JS array, and from there the JS
code picks it up.