test: fix failure in feature_nulldummy.py on single-core machines
test: fix failure in feature_nulldummy.py on single-core machines

On single-core machines, executing the test feature_nulldummy.py results in
the following assertion error:

...
2021-08-18T15:37:58.805000Z TestFramework (INFO): Test 4: Non-NULLDUMMY base multisig transaction is invalid after activation
2021-08-18T15:37:58.814000Z TestFramework (ERROR): Assertion failed
Traceback (most recent call last):
  File "[...]/test/functional/test_framework/test_framework.py", line 131, in main
    self.run_test()
  File "[...]/test/functional/feature_nulldummy.py", line 107, in run_test
    self.block_submit(self.nodes[0], [test4tx], accept=False)
  File "[...]/test/functional/feature_nulldummy.py", line 134, in block_submit
    assert_equal(None if accept else 'block-validation-failed', node.submitblock(block.serialize().hex()))
  File "[...]/test/functional/test_framework/util.py", line 49, in assert_equal
    raise AssertionError("not(%s)" % " == ".join(str(arg) for arg in (thing1, thing2) + args))
AssertionError: not(block-validation-failed == non-mandatory-script-verify-flag (Dummy CHECKMULTISIG argument must be zero))
2021-08-18T15:37:58.866000Z TestFramework (INFO): Stopping nodes
...

The behaviour can be reproduced on a multi-core machine by simply changing the
function GetNumCores() (in src/util/system.cpp) to return 1:

int GetNumCores()
{
    return 1;
}

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