de85af5cce test: store subversion (user agent) as string in msg_version (Sebastian Falbesoner)
Pull request description:
It seems more natural to treat the "subversion" field (=user agent string, see [BIP 14](https://github.com/bitcoin/bips/blob/master/bip-0014.mediawiki#Proposal)) of a node as pure string rather than a bytestring within the test framework. This is also suggested with the naming prefix in `msg_version.strSubVer`: one probably wouldn't expect a field starting with "str" to be a bytestring that needs further decoding to be useful. This PR moves the encoding/decoding parts to the serialization/deserialization routines so that the user doesn't have to bother with that anymore.
Note that currently, in the master branch the `msg_version.strSubVer` is never read (only in `msg_version.__repr__`); However, one issue that is solved by this PR came up while testing #19509 (not merged yet): A decoding script for binary message capture files takes use of the functional test framework convert it into JSON format. Bytestrings will be convered to hexstrings, while pure strings will (surprise surprise) end up without modification in the file.
So without this patch, we get:
```
$ jq . out.json | grep -m5 strSubVer
"strSubVer": "2f5361746f7368693a32312e39392e302f"
"strSubVer": "2f5361746f7368693a302e32302e312f"
"strSubVer": "2f5361746f7368693a32312e39392e302f"
"strSubVer": "2f5361746f7368693a302e32302e312f"
"strSubVer": "2f5361746f7368693a32312e39392e302f"
```
After this patch:
```
$ jq . out2.json | grep -m5 strSubVer
"strSubVer": "/Satoshi:21.99.0/"
"strSubVer": "/Satoshi:0.20.1/"
"strSubVer": "/Satoshi:21.99.0/"
"strSubVer": "/Satoshi:0.20.1/"
"strSubVer": "/Satoshi:21.99.0/"
```
ACKs for top commit:
jnewbery:
utACK de85af5cce
Tree-SHA512: ff23642705c858e8387a625537dfec82e6b8a15da6d99b8d12152560e52d243ba17431b602b26f60996d897e00e3f37dcf8dc8a303ffb1d544df29a5937080f9
69f7f50aa5