Improve feature test support

[rbri]

• enhance Utils a bit to support feature testing
• migrate a feature test case from core-js to rhino (found while updating to the latest rhino source)

[andreabergia]

This looks great! Removing all those various context factory makes it quite a bit more readable.

I would suggest a slightly different API name though, i.e. rather than

        factory = new Utils.FeatureContextFactory(Context.FEATURE_DYNAMIC_SCOPE);

have something like:

        factory = Utils.contextFactoryWithFeatures(Context.FEATURE_DYNAMIC_SCOPE);

[rbri]

@andreabergia good point, will do that update

[gbrail]

Looks good to me -- thanks!