add TestDecompressorUNPACKZdate2() test for unpacking
add TestDecompressorUNPACKZdate2() test for unpacking

tpukep/caps,hodduc/go-capnproto2,zombiezen/go-capnproto,zombiezen/go-capnproto2,hodduc/go-capnproto2,zombiezen/go-capnproto,zombiezen/go-capnproto2