add unit tests for the decodescript rpc

[mruddy]

Adding unit tests for the "decodescript" RPC because:

1. I didn't see any tests specifically for the "decodescript" RPC.
2. 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 Mempool only CHECKLOCKTIMEVERIFY (BIP65) verification, unparameterized version #6124 (comment).
3. These will be good to have if/when I get back around to show scriptSig signature hash types in transaction decodes. fixes #3166 #5264 and Remove the "OP_" prefix from transaction script decodes. #5392.

[jgarzik]

ut ACK

[laanwj]

Concept ACK. Thanks for adding tests.

However, wouldn't these be more appropriate in src/qa/rpc-tests/rawtransactions.py?

I suppose it doesn't make much of a difference as this is a stateless utility function.

[mruddy]

@laanwj Thanks for the feedback. Yes, I suppose it would make sense to move these over into the python tests. They are more integration level tests anyways. I just did them this way because I had some other changes and could run everything with a single "make check". I'll look at moving these over later today or tomorrow.

[mruddy]

@laanwj Updated -- I decided to use a separate new decodescript.py file to avoid muddling the existing rawtransactions.py. I also added a short snippet to the main README referencing this set of RPC tests.

[laanwj]

ACK