qa: split coins in a single tx in fee estimation test
qa: split coins in a single tx in fee estimation test

This simplifies the code, and slightly speeds up the test.

Running `./test/functional/test_runner.py -j15 $(printf 'feature_fee_estimation %.0s' {1..15})`
on master 3 times gives:

- Before:
    ALL                       | ✓ Passed  | 788 s (accumulated)
    ALL                       | ✓ Passed  | 818 s (accumulated)
    ALL                       | ✓ Passed  | 873 s (accumulated)

- After:
    ALL                       | ✓ Passed  | 763 s (accumulated)
    ALL                       | ✓ Passed  | 798 s (accumulated)
    ALL                       | ✓ Passed  | 731 s (accumulated)

Signed-off-by: Antoine Poinsot <0fdf30a351670cde318d282b5fede3c3548275bc@protonmail.com>

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