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

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

The new format should be equivalent to the old format, and it is now the
default format when running the libc++ tests. This commit changes the
libc++abi tests to use the new format by default too. If unexpected failures
are discovered, it should be fine to revert this commit until they are
addressed.

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.