Explicitly unittest the canonical prunable txout format

Test every invalid op code, and add an explicit test for Gavin's proposed format for provably unspendable/always prunable transactions. Specifically any scriptPubKey of the form with 0xFF: (one of the invalid op codes)

    0xFF {zero or more ops}

Because the scriptSig and scriptPubKey are processed separately one can't even use IF/ENDIF trickery to defeat this. I also like how it says "INVALID" quite clearly as the first opcode.

Unless anyone has a reason why this standard is a bad idea I'm also going to raise the issue with P2Pool; currently they are the main producer of non-prunable txouts that could be made prunable.

No code to actually do anything with this standard is included in this patch; that can be done later.