Merge bitcoin/bitcoin#22257: test: refactor: various (de)serialization helpers cleanups/improvements

bdb8b9a347 test: doc: improve doc for `from_hex` helper (mention `to_hex` alternative) (Sebastian Falbesoner)
1914054208 scripted-diff: test: rename `FromHex` to `from_hex` (Sebastian Falbesoner)
a79396fe5f test: remove `ToHex` helper, use .serialize().hex() instead (Sebastian Falbesoner)
2ce7b47958 test: introduce `tx_from_hex` helper for tx deserialization (Sebastian Falbesoner)

Pull request description:

  There are still many functional tests that perform conversions from a hex-string to a message object (deserialization) manually. This PR identifies all those instances and replaces them with a newly introduced helper `tx_from_hex`.

  Instances were found via
  * `git grep "deserialize.*BytesIO"`

  and some of them manually, when it were not one-liners.

  Further, the helper `ToHex` was removed and simply replaced by `.serialize().hex()`, since now both variants are in use (sometimes even within the same test) and using the helper doesn't really have an advantage in readability. (see discussion https://github.com/bitcoin/bitcoin/pull/22257#discussion_r652404782)

ACKs for top commit:
  MarcoFalke:
    review re-ACK bdb8b9a347 😁

Tree-SHA512: e25d7dc85918de1d6755a5cea65471b07a743204c20ad1c2f71ff07ef48cc1b9ad3fe5f515c1efaba2b2e3d89384e7980380c5d81895f9826e2046808cd3266e

test/functional/test_framework/blocktools.py

@@ -23,12 +23,11 @@ from .messages import (
     CTxIn,
     CTxInWitness,
     CTxOut,
-    FromHex,
-    ToHex,
     hash256,
     hex_str_to_bytes,
     ser_uint256,
     sha256,
+    tx_from_hex,
     uint256_from_str,
 )
 from .script import (
@@ -79,7 +78,7 @@ def create_block(hashprev=None, coinbase=None, ntime=None, *, version=None, tmpl
     if txlist:
         for tx in txlist:
             if not hasattr(tx, 'calc_sha256'):
-                tx = FromHex(CTransaction(), tx)
+                tx = tx_from_hex(tx)
             block.vtx.append(tx)
     block.hashMerkleRoot = block.calc_merkle_root()
     block.calc_sha256()
@@ -166,7 +165,7 @@ def create_transaction(node, txid, to_address, *, amount):
         sign for the output that is being spent.
     """
     raw_tx = create_raw_transaction(node, txid, to_address, amount=amount)
-    tx = FromHex(CTransaction(), raw_tx)
+    tx = tx_from_hex(raw_tx)
     return tx
 
 def create_raw_transaction(node, txid, to_address, *, amount):
@@ -243,9 +242,9 @@ def send_to_witness(use_p2wsh, node, utxo, pubkey, encode_p2sh, amount, sign=Tru
         return node.sendrawtransaction(signed["hex"])
     else:
         if (insert_redeem_script):
-            tx = FromHex(CTransaction(), tx_to_witness)
+            tx = tx_from_hex(tx_to_witness)
             tx.vin[0].scriptSig += CScript([hex_str_to_bytes(insert_redeem_script)])
-            tx_to_witness = ToHex(tx)
+            tx_to_witness = tx.serialize().hex()
 
     return node.sendrawtransaction(tx_to_witness)