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/feature_coinstatsindex.py

@@ -22,7 +22,6 @@ from test_framework.messages import (
     CTransaction,
     CTxIn,
     CTxOut,
-    ToHex,
 )
 from test_framework.script import (
     CScript,
@@ -170,7 +169,7 @@ class CoinStatsIndexTest(BitcoinTestFramework):
         tx2 = CTransaction()
         tx2.vin.append(CTxIn(COutPoint(int(tx1_txid, 16), n), b''))
         tx2.vout.append(CTxOut(int(20.99 * COIN), CScript([OP_RETURN] + [OP_FALSE]*30)))
-        tx2_hex = self.nodes[0].signrawtransactionwithwallet(ToHex(tx2))['hex']
+        tx2_hex = self.nodes[0].signrawtransactionwithwallet(tx2.serialize().hex())['hex']
         self.nodes[0].sendrawtransaction(tx2_hex)
 
         # Include both txs in a block
@@ -207,7 +206,7 @@ class CoinStatsIndexTest(BitcoinTestFramework):
         block_time = self.nodes[0].getblock(tip)['time'] + 1
         block = create_block(int(tip, 16), cb, block_time)
         block.solve()
-        self.nodes[0].submitblock(ToHex(block))
+        self.nodes[0].submitblock(block.serialize().hex())
         self.sync_all()
 
         self.wait_until(lambda: not try_rpc(-32603, "Unable to read UTXO set", index_node.gettxoutsetinfo, 'muhash'))