Add isctest.transfer.transfer_message() helper and convert tests

Add isctest.transfer.transfer_message() helper and convert tests

Add a new helper function, isctest.transfer.transfer_message(), to
bin/tests/system/isctest/transfer.py that generates the log message
produced by xfrin_log() in lib/dns/xfrin.c for an incoming zone
transfer:

    transfer of '<zone>/IN' from <source_ns>#<port>: <msg>

The helper always returns a compiled re.Pattern.  source_ns and port
each accept None to match any source address / port.  msg accepts
either a plain str (regex-escaped automatically) or a compiled
re.Pattern (spliced into the regex as-is), so callers that need regex
syntax in the message part can pass Re(r"...") without having to
wrap the whole result.

source_ns is passed through re.escape() when provided, so dots in
IPv4 addresses (e.g. "10.53.0.1") match a literal dot rather than
any character.

Convert the existing call sites across the system tests to use the
new helper.

Co-Authored-By: Nicki Křížek <nicki@isc.org>
Assisted-by: Claude:claude-sonnet-4-6
Assisted-by: Claude:claude-opus-4-7