Adding unit tests for the "decodescript" RPC because:
- I didn't see any tests specifically for the "decodescript" RPC.
- In adding these, I added a case that will fail until fixed if/when the OP_NOP2 name decode changes to OP_CHECKLOCKTIMEVERIFY are made. Basically a minor follow-up to #6124 (comment).
- These will be good to have if/when I get back around to #5264 and #5392.