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 #22257 (review))