The helper `send_large_txs` in its current interface has a fee_rate
parameter, implying that it would create a transaction with exactly that
rate. Unfortunately, this fee rate is only passed to MiniWallet's
`create_self_transfer` method, which can't know that we append several
tx outputs after, increasing the tx's vsize and decreasing it's fee rate
accordingly.

In our case, the fee rate is off by several orders of magnitude, as the
tx's vsize changes changes from 96 to 67552 vbytes (>700x), i.e. the
value passed to this function is neither really a fee rate nor an
absolute fee, but something in-between, which is very confusing.

Clarify the interface by passing an absolute fee that is deducted in the end
(and verified, via testmempoolaccept) and also describe how we come up with the
value passed.

./test/functional/test_runner.py mempool_limit