test: MiniWallet: skip mempool check if `mempool_valid=False`

MiniWallet's core method for creating txs (`create_self_transfer`)
right now always executes the `testmempoolaccept` RPC to check for
mempool validity or invalidity. In some test cases where we use
MiniWallet to create a huge number of transactions this can lead
to performance issues (e.g. feature_fee_estimation.py where the
execution time after MiniWallet usage almost doubled). Providing
the possibility to skip the mempool checks is a mitigation for
this.

master branch:
$ time ./test/functional/feature_fee_estimation.py
real    3m20.771s
user    2m52.360s
sys     0m39.340s

PR branch:
$ time ./test/functional/feature_fee_estimation.py
real    2m1.386s
user    1m42.510s
sys     0m22.980s

test/functional/test_framework/wallet.py

@@ -234,7 +234,8 @@ class MiniWallet:
         return tx
 
     def create_self_transfer(self, *, fee_rate=Decimal("0.003"), from_node=None, utxo_to_spend=None, mempool_valid=True, locktime=0, sequence=0):
-        """Create and return a tx with the specified fee_rate. Fee may be exact or at most one satoshi higher than needed."""
+        """Create and return a tx with the specified fee_rate. Fee may be exact or at most one satoshi higher than needed.
+           Checking mempool validity via the testmempoolaccept RPC can be skipped by setting mempool_valid to False."""
         from_node = from_node or self._test_node
         utxo_to_spend = utxo_to_spend or self.get_utxo()
         if self._priv_key is None:
@@ -261,12 +262,13 @@ class MiniWallet:
             tx.wit.vtxinwit[0].scriptWitness.stack = [CScript([OP_TRUE]), bytes([LEAF_VERSION_TAPSCRIPT]) + self._internal_key]
         tx_hex = tx.serialize().hex()
 
-        tx_info = from_node.testmempoolaccept([tx_hex])[0]
-        assert_equal(mempool_valid, tx_info['allowed'])
         if mempool_valid:
+            tx_info = from_node.testmempoolaccept([tx_hex])[0]
+            assert_equal(tx_info['allowed'], True)
             assert_equal(tx_info['vsize'], vsize)
             assert_equal(tx_info['fees']['base'], utxo_to_spend['value'] - Decimal(send_value) / COIN)
-        return {'txid': tx_info['txid'], 'wtxid': tx_info['wtxid'], 'hex': tx_hex, 'tx': tx}
+
+        return {'txid': tx.rehash(), 'wtxid': tx.getwtxid(), 'hex': tx_hex, 'tx': tx}
 
     def sendrawtransaction(self, *, from_node, tx_hex, **kwargs):
         txid = from_node.sendrawtransaction(hexstring=tx_hex, **kwargs)