Add erroroffset support to perltest.sh and fix discrepancies

[NWilson]

• perltest.sh would need to support outputting error offsets from the Perl messages
• RunPerlTest would need to perform a "loose" diff on certain lines, to handle the fact that the Perl error messages won't have matching error codes
• Migrate as many error cases as possible from testinput{2,5} → testinput{1,4}
• Fix as many discrepancies as possible between the error offsets reported by Perl and PCRE2

There's no particular urgency to this. It would be nice though, perhaps when I have spare time and all the other more urgent tasks on my list are done.