APItest/t/utf8_warn_base: Add tests

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 unexpected 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.

ext/XS-APItest/t/utf8_warn_base.pl

@@ -1190,7 +1190,12 @@ ($)
  # We try various combinations of malformations that can occur
  foreach my $short (0, 1) {
  next if $skip_most_tests && $short;
- foreach my $unexpected_noncont (0, 1) {
+ # Insert an unexpected non-continuation in every possible position
+ my $unexpected_noncont;
+ for ($unexpected_noncont = $length - $short - 1;
+ $unexpected_noncont > 0;
+ $unexpected_noncont--)
+ {
  next if $skip_most_tests && $unexpected_noncont;
  foreach my $overlong (0, 1) {
  next if $overlong && $skip_most_tests;
@@ -1318,11 +1323,14 @@ ($)
 
  if ($unexpected_noncont) {
 
- # To force this malformation, change the final continuation
- # byte into a start byte.
- my $pos = ($short) ? -2 : -1;
- substr($this_bytes, $pos, 1) = $known_start_byte;
- $this_expected_len--;
+ # The overlong tweaking above changes the first bytes to
+ # specified values; we better not override those.
+ next if $overlong;
+
+ # To force this malformation, change a continuation byte into a
+ # start byte.
+ substr($this_bytes, $unexpected_noncont, 1) = $known_start_byte;
+ $this_expected_len = $unexpected_noncont;
  }
 
  # The whole point of a test that is malformed from the beginning