[libc++] 2/N: Enable the new libc++ testing format by default

Both test formats are equivalent, so this *should* not be a problem.
We've fixed a couple of failures uncovered by the first time we tried
making the switch, so this new attempt should go even farther.

If failures are noticed, it should be fine to revert this commit, but
please give a heads up afterwards so we know to address the issues!

Also note that it is still possible to use the old format by passing
`--param=use_old_format=True` when running Lit for the time being.