[libc++] Enable the new libc++ testing format by default

Both test formats are equivalent, so this *should* not be a problem.
However, I'm taking advantage of the week-end to test this and see if
there are any failures. If so, it should be fine to revert this until
the failures have been addressed.

For the time being, it is still possible to use the old format by passing
`--param=use_old_format=True` when running Lit.