[PowerPC][NFC] Rename/organize encoding test files for ISA3.1

Rename `future*` encoding test files to include ISA3.1 in the file name
and combine with exisitng ISA3.1 instruction encoding tests that were
added into `p10*` test files.

Keeping the `p10*` files for now to ensure we don't add more to it.
Will remove once all ISA3.1 instruction are implemented.