APItest/t/utf8_warn_base: Add tests

[khwilliamson]

One UTF-8 malformation is when the string has a start byte in it before the expected end of the character. This test file tested the case where the unexpected byte came in the final position. GH #22597 found bugs where the undexpected byte came immediately after the first byte.

This commit adds tests for unexpected bytes in all possible positions. if the fix for GH #22597 is reverted, this new revised file has 1400 failures.

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• This set of changes does not require a perldelta entry.

[jkeenan]

The following test failed in one (but only one) of our CI configurations: ext/XS-APItest/t/utf8_warn00.t. See https://github.com/Perl/perl5/actions/runs/11241805335/job/31254645122?pr=22646. However, when I tried to reproduce this locally I could not.

I subsequently checked out this pull request and built an unthreaded perl. At the end of make test_prep I got these build-time warnings:

cc -c   -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -Wall -Werror=pointer-arith -Werror=vla -Wextra -Wno-long-long -Wno-declaration-after-statement -Wc++-compat -Wwrite-strings -O2   -DVERSION=\"1.32\" -DXS_VERSION=\"1.32\" -fPIC "-I../.."   Normalize.c
In file included from ../../perl.h:7895,
                 from Normalize.xs:13:
../../inline.h: In function ‘Perl_is_utf8_string_flags’:
../../inline.h:2090:41: warning: bitwise comparison always evaluates to false [-Wtautological-compare]
 2090 |                                         == UTF8_DISALLOW_ILLEGAL_INTERCHANGE)
      |                                         ^~
../../inline.h:2096:40: warning: bitwise comparison always evaluates to false [-Wtautological-compare]
 2096 |                                        == UTF8_DISALLOW_ILLEGAL_C9_INTERCHANGE)
      |                                        ^~
../../inline.h: In function ‘Perl_is_utf8_string_loclen_flags’:
../../inline.h:2527:41: warning: bitwise comparison always evaluates to false [-Wtautological-compare]
 2527 |                                         == UTF8_DISALLOW_ILLEGAL_INTERCHANGE)
      |                                         ^~
../../inline.h:2533:37: warning: bitwise comparison always evaluates to false [-Wtautological-compare]
 2533 |                                     == UTF8_DISALLOW_ILLEGAL_C9_INTERCHANGE)
      |                                     ^~
rm -f ../../lib/auto/Unicode/Normalize/Normalize.so
cc  -shared -O2 -L/usr/local/lib -fstack-protector-strong  Normalize.o  -o ../../lib/auto/Unicode/Normalize/Normalize.so  \
      \
  
chmod 755 ../../lib/auto/Unicode/Normalize/Normalize.so
make[1]: Leaving directory '/home/jkeenan/gitwork/perl2/dist/Unicode-Normalize'
./perl -Ilib -I. -f pod/buildtoc -q
cd t && (rm -f perl; /usr/bin/ln -s ../perl perl)