Fixes#967. In this test, if the randomized fee calculated from the
tx_fees field in the config happens to be at the bottom of the range,
it's possible that the calculated feerate is very close to 1.1 sats/vB,
but this can still vary slightly due to signature sizes, resulting in a
very rare case where this test could erroneously fail.
This is fixed by bumping the lowest minfeerate by 2%.