test: simplify and document NULLDUMMY-invalidation helper
test: simplify and document NULLDUMMY-invalidation helper

The function `trueDummy` in feature_nulldummy.py is currently more
complicated than it needs to be. Rather than converting the scriptSig to
a CScript and looping through it to build a new scriptSig with the
modified push, simply directly replace the push of null (OP_0) with a
push of one (OP_TRUE/OP_1).

Note that on master, actually an element with the value of 0x51 is
pushed (0x0151...) -- this was very likely not intended, as 0x51 is the
script op-code for OP_TRUE, and also the function's name suggests that
the "true" value shall be pushed.

andreaskern/bitcoin,domob1812/namecore,pataquets/namecoin-core,GroestlCoin/bitcoin,jambolo/bitcoin,bitcoinknots/bitcoin,ajtowns/bitcoin,dscotese/bitcoin,namecoin/namecoin-core,mm-s/bitcoin,achow101/bitcoin,particl/particl-core,domob1812/bitcoin,mm-s/bitcoin,sstone/bitcoin,jamesob/bitcoin,jlopp/statoshi,jamesob/bitcoin,sipsorcery/bitcoin,jlopp/statoshi,AkioNak/bitcoin,tecnovert/particl-core,GroestlCoin/bitcoin,lateminer/bitcoin,sipsorcery/bitcoin,anditto/bitcoin,sstone/bitcoin,bitcoinknots/bitcoin,lateminer/bitcoin,sipsorcery/bitcoin,anditto/bitcoin,Xekyo/bitcoin,Xekyo/bitcoin,domob1812/namecore,namecoin/namecore,anditto/bitcoin,particl/particl-core,andreaskern/bitcoin,Xekyo/bitcoin,fanquake/bitcoin,bitcoinsSG/bitcoin,andreaskern/bitcoin,mm-s/bitcoin,dscotese/bitcoin,GroestlCoin/bitcoin,achow101/bitcoin,sstone/bitcoin,MarcoFalke/bitcoin,kallewoof/bitcoin,jamesob/bitcoin,anditto/bitcoin,dscotese/bitcoin,tecnovert/particl-core,dscotese/bitcoin,jamesob/bitcoin,andreaskern/bitcoin,fujicoin/fujicoin,dscotese/bitcoin,sstone/bitcoin,instagibbs/bitcoin,ajtowns/bitcoin,instagibbs/bitcoin,GroestlCoin/GroestlCoin,domob1812/namecore,GroestlCoin/GroestlCoin,bitcoinsSG/bitcoin,anditto/bitcoin,GroestlCoin/bitcoin,anditto/bitcoin,bitcoin/bitcoin,particl/particl-core,domob1812/namecore,fujicoin/fujicoin,bitcoinknots/bitcoin,mruddy/bitcoin,MarcoFalke/bitcoin,fanquake/bitcoin,tecnovert/particl-core,AkioNak/bitcoin,sipsorcery/bitcoin,sipsorcery/bitcoin,ajtowns/bitcoin,GroestlCoin/bitcoin,mruddy/bitcoin,lateminer/bitcoin,namecoin/namecoin-core,fujicoin/fujicoin,jambolo/bitcoin,fanquake/bitcoin,Xekyo/bitcoin,tecnovert/particl-core,mruddy/bitcoin,MarcoFalke/bitcoin,particl/particl-core,domob1812/namecore,kallewoof/bitcoin,mm-s/bitcoin,achow101/bitcoin,namecoin/namecore,jambolo/bitcoin,MarcoFalke/bitcoin,namecoin/namecore,MarcoFalke/bitcoin,jlopp/statoshi,AkioNak/bitcoin,fujicoin/fujicoin,ajtowns/bitcoin,fanquake/bitcoin,mm-s/bitcoin,bitcoinknots/bitcoin,ajtowns/bitcoin,particl/particl-core,domob1812/namecore,Xekyo/bitcoin,instagibbs/bitcoin,AkioNak/bitcoin,mruddy/bitcoin,achow101/bitcoin,jlopp/statoshi,AkioNak/bitcoin,bitcoinsSG/bitcoin,pataquets/namecoin-core,bitcoinknots/bitcoin,andreaskern/bitcoin,tecnovert/particl-core,sstone/bitcoin,sipsorcery/bitcoin,kallewoof/bitcoin,bitcoin/bitcoin,lateminer/bitcoin,fanquake/bitcoin,kallewoof/bitcoin,instagibbs/bitcoin,AkioNak/bitcoin,namecoin/namecore,lateminer/bitcoin,fanquake/bitcoin,instagibbs/bitcoin,GroestlCoin/GroestlCoin,bitcoin/bitcoin,namecoin/namecoin-core,achow101/bitcoin,mm-s/bitcoin,jlopp/statoshi,namecoin/namecore,instagibbs/bitcoin,namecoin/namecoin-core,jlopp/statoshi,tecnovert/particl-core,namecoin/namecore,GroestlCoin/bitcoin,mruddy/bitcoin,fujicoin/fujicoin,sstone/bitcoin,namecoin/namecoin-core,domob1812/bitcoin,bitcoin/bitcoin,pataquets/namecoin-core,GroestlCoin/GroestlCoin,bitcoin/bitcoin,jambolo/bitcoin,MarcoFalke/bitcoin,bitcoinsSG/bitcoin,pataquets/namecoin-core,pataquets/namecoin-core,particl/particl-core,jamesob/bitcoin,GroestlCoin/GroestlCoin,andreaskern/bitcoin,bitcoinsSG/bitcoin,domob1812/bitcoin,fujicoin/fujicoin,domob1812/bitcoin,bitcoin/bitcoin,jambolo/bitcoin,kallewoof/bitcoin,pataquets/namecoin-core,lateminer/bitcoin,dscotese/bitcoin,achow101/bitcoin,namecoin/namecoin-core,jambolo/bitcoin,jamesob/bitcoin,ajtowns/bitcoin,kallewoof/bitcoin,bitcoinsSG/bitcoin,domob1812/bitcoin,Xekyo/bitcoin,domob1812/bitcoin,mruddy/bitcoin,GroestlCoin/GroestlCoin