t/loc_tools.pl: Update text for searched for diagnostic

This fixes #21113.

This script looks for locales that have no idiosyncracies.  It excludes
ones that generate warnings that there are potential issues with them.
However, commit 667fa5f changed the
warning text, without updating this script with the new text.  Hence,
problematic locales were let through.

Simply update the searched-for text now, and add a cross-referencing
comment to where the text is generated to keep this from happening
again.

The problem in this ticket is that a locale chosen on this platform
happens to have idiosyncracies.  Other platforms happen to currently
choose different locales that aren't problematic.  This change keeps any
problematic locale from being chosen.

locale.c

@@ -2424,6 +2424,9 @@ S_new_ctype(pTHX_ const char *newctype, bool force)
         if (   (UNLIKELY(bad_count))
             && (LIKELY(ckWARN_d(WARN_LOCALE)) || UNLIKELY(DEBUG_L_TEST)))
         {
+            /* WARNING.  If you change the wording of these; be sure to update
+             * t/loc_tools.pl correspondingly */
+
             if (PL_in_utf8_CTYPE_locale) {
                 PL_warn_locale = Perl_newSVpvf(aTHX_
                      "Locale '%s' contains (at least) the following characters"

t/loc_tools.pl

@@ -180,7 +180,7 @@ ($$$$)
         $badutf8 = 1 if grep { /Malformed UTF-8/ } @_;
         $unsupported = 1 if grep { /Locale .* is unsupported/i } @_;
         $plays_well = 0 if grep {
-                    /Locale .* may not work well(?#
+                    /The following characters .* may not have the same meaning as the Perl program expects(?#
                    )|The Perl program will use the expected meanings/i
             } @_;
     };