In order to enable exact fee calculation for transactions that spend
P2PK outputs in the MiniWallet, we enforce the created signatures to
have a fixed length (>49.89% probability) by default. With that it is
easier to check the created transactions vsize and avoid flaky tests
that would appear whenever the signatures R- or S-values are smaller
(due to leading zero bytes).
Note that to get the total scriptSig size one has to add another
2 bytes, as there is also the OP_PUSHx instruction on the front and
the sighash type byte on the back, leading to a final scriptSig size
of 73 bytes.
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