The test harness is compatible with bitcoin-core 0.19, 0.20, 0.21.
Also removed some unnecessary comments about registering dispute
agents in the test harness because it now happens by default.
The codacy issue "$/${} is unnecessary on arithmetic variables"
seems to not apply to $(echo $((1 + $RANDOM % 10))), but we
try another syntax to make her happy.
The script demonstrats a way to always keep one offer in the market.
As soon as the offer is taken and the trade is completed, another
offer is placed.
