test: apply fixed feerate to avoid variable dynamic fees
test: apply fixed feerate to avoid variable dynamic fees

Without specifying a feerate, we let the wallet decide on an
appropriate feerate, which can be influenced by various factors
such as what's in the mempool. Since wallet_groups.py fails when
feerates are unstable, we should use a fixed feerate across all nodes.

Closes #25940

mruddy/bitcoin,bitcoin/bitcoin,tecnovert/particl-core,fanquake/bitcoin,AkioNak/bitcoin,ajtowns/bitcoin,bitcoinsSG/bitcoin,AkioNak/bitcoin,fujicoin/fujicoin,fanquake/bitcoin,ajtowns/bitcoin,mruddy/bitcoin,fujicoin/fujicoin,sipsorcery/bitcoin,particl/particl-core,kallewoof/bitcoin,jambolo/bitcoin,tecnovert/particl-core,lateminer/bitcoin,AkioNak/bitcoin,ajtowns/bitcoin,namecoin/namecoin-core,namecoin/namecoin-core,sstone/bitcoin,bitcoinsSG/bitcoin,fujicoin/fujicoin,lateminer/bitcoin,namecoin/namecoin-core,sipsorcery/bitcoin,mruddy/bitcoin,namecoin/namecore,jambolo/bitcoin,lateminer/bitcoin,kallewoof/bitcoin,sipsorcery/bitcoin,namecoin/namecoin-core,AkioNak/bitcoin,jambolo/bitcoin,particl/particl-core,bitcoinsSG/bitcoin,kallewoof/bitcoin,sipsorcery/bitcoin,mruddy/bitcoin,tecnovert/particl-core,lateminer/bitcoin,jambolo/bitcoin,lateminer/bitcoin,sstone/bitcoin,kallewoof/bitcoin,kallewoof/bitcoin,bitcoinsSG/bitcoin,fanquake/bitcoin,namecoin/namecore,fanquake/bitcoin,AkioNak/bitcoin,ajtowns/bitcoin,ajtowns/bitcoin,particl/particl-core,bitcoin/bitcoin,AkioNak/bitcoin,jambolo/bitcoin,ajtowns/bitcoin,bitcoinsSG/bitcoin,particl/particl-core,namecoin/namecore,namecoin/namecore,jambolo/bitcoin,fujicoin/fujicoin,particl/particl-core,particl/particl-core,tecnovert/particl-core,fanquake/bitcoin,bitcoin/bitcoin,mruddy/bitcoin,bitcoin/bitcoin,tecnovert/particl-core,sstone/bitcoin,sipsorcery/bitcoin,kallewoof/bitcoin,bitcoinsSG/bitcoin,tecnovert/particl-core,namecoin/namecoin-core,namecoin/namecore,sipsorcery/bitcoin,bitcoin/bitcoin,namecoin/namecoin-core,fujicoin/fujicoin,mruddy/bitcoin,fujicoin/fujicoin,sstone/bitcoin,namecoin/namecore,sstone/bitcoin,bitcoin/bitcoin,sstone/bitcoin,lateminer/bitcoin,fanquake/bitcoin