This adds FindAndDelete tests for pre-segwit and segwit scripts, to show that FindAndDelete is performed only in pre-segwit scripts. The transaction is generated in an unconventional way: the public key is generated with key recovery, using a fixed signature (committed in prevout) and a known sighash.
Also edited the comments in interpreter.cpp since the rationale of dropping the signature was wrong.
No harm to backport but I don't feel this is needed. This is more important for alt-implementations to make sure they won't fork accidentally by incorrectly implementing BIP143.
