test: MiniWallet: always rehash after signing (P2PK mode)

Also explicitly rehash in the cases where we modify a tx after signing
in feature_csv_activation.py. Parts of this test relied on the fact that
rehashing of transactions is done in the course of calculating a block's
merkle root (`calc_merkle_root`), which only works if no hash was
calculated before due to a caching mechanism.

In the following commit the txid in MiniWallet is calculated via
`rehash()`, i.e. this doesn't work anymore and we always have to
explicitely have the right hash before we calculate the merkle root.

test/functional/test_framework/wallet.py

@@ -127,6 +127,7 @@ class MiniWallet:
             if not fixed_length:
                 break
         tx.vin[0].scriptSig = CScript([der_sig + bytes(bytearray([SIGHASH_ALL]))])
+        tx.rehash()
 
     def generate(self, num_blocks, **kwargs):
         """Generate blocks with coinbase outputs to the internal address, and append the outputs to the internal list"""