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

[libunwind] 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++ and libc++abi tests. This commit
changes the libunwind 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.

Differential Revision: https://reviews.llvm.org/D77733