[test] Round target fee to 8 decimals in assert_fee_amount
Summary:
The output would produce arbitrary number of decimal points, sometimes resulting in 9 decimals: AssertionError: Fee of 0.00000415 BTC too low! (Should be 0.000006175 BTC) The above looks like the expected fee is 6175 sats when in reality it's 618.
Backport of core PR12486
https://github.com/bitcoin/bitcoin/pull/12486/files
Test Plan:
./test/functional/test_runner.py
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D3516