test_framework: Avoid infinite loop in encoding Decimal
test_framework: Avoid infinite loop in encoding Decimal

Avoid an infinite loop in encoding, by ensuring EncodeDecimal
returns a string. round(Decimal) used to convert it to
float, but it no longer does in python 3.x. Strings are
supported since #6380, so just use that.

bitcoinxt/bitcoinxt,bitcoinxt/bitcoinxt,dagurval/bitcoinxt,dagurval/bitcoinxt,bitcoinxt/bitcoinxt,dagurval/bitcoinxt,dagurval/bitcoinxt,dagurval/bitcoinxt,bitcoinxt/bitcoinxt,bitcoinxt/bitcoinxt,bitcoinxt/bitcoinxt,dagurval/bitcoinxt