Test unexecuted OP_CODESEPARATOR

OP_CODESEPARATOR is an actual executed instruction, not a declarative
thing, so if it's wrapped in an OP_IF it can be turned off.

Using this to implement Rivest's Paywords is left as an exercise for the
reader.
Merge pull request #5421

cac15be Test unexecuted OP_CODESEPARATOR (Peter Todd)