test: Fix intermitent failure in wallet_basic.py #23141

assert_fee_amount currently uses round() when calculating the fee rate, which causes intermittent failure in wallet_basic.py: https://cirrus-ci.com/task/5721255786577920?logs=ci#L145

2021-09-29T23:28:41.198000Z TestFramework (INFO): Test sendtoaddress with fee_rate param (explicit fee rate in sat/vB)
2021-09-29T23:28:49.992000Z TestFramework (ERROR): Assertion failed
Traceback (most recent call last):
  File "/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-pc-linux-gnu/test/functional/test_framework/test_framework.py", line 131, in main
    self.run_test()
  File "/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-pc-linux-gnu/test/functional/wallet_basic.py", line 500, in run_test
    assert_fee_amount(fee, tx_size, Decimal(fee_rate_btc_kvb))
  File "/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-pc-linux-gnu/test/functional/test_framework/util.py", line 41, in assert_fee_amount
    raise AssertionError("Fee of %s BTC too low! (Should be %s BTC)" % (str(fee), str(target_fee)))
AssertionError: Fee of 0.00000255 BTC too low! (Should be 0.00000256 BTC)

We should truncate rather than rounding.

This would also be fixed by #22949 but it makes more sense to make the test consistent with the current code.

See #23136.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #23136 (test: update fee rate assertion helper in the functional test framework by jonatack)
• #22364 (wallet: Make a tr() descriptor by default by achow101)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Closing as duplicate of https://github.com/bitcoin/bitcoin/pull/23136