test: Fix P2WPKH_FAKE_WITNESS_SIZE

test: Fix P2WPKH_FAKE_WITNESS_SIZE

We would previously push 108 bytes on a P2WPKH witness
to simulate signature + pubkey. This was wrong: we should push
106 bytes instead.
The max satisfaction size for a P2WPKH is 112 WU:
elements in witness (1 byte, 1WU) + OP_PUSH (1 byte, 1WU) +
pk (33 bytes, 33 WU) + OP_PUSH (1 byte, 1WU) + signature and sighash
(72 bytes, 72 WU) + scriptsig len (1 byte, 4WU)
We should push on the witness pk + signature and sighash. This is 105
WU. Since we push just once instead of twice, we add 1WU for the OP_PUSH
we are omitting.