tests: let yacc's yyerror take its argument as a const string

tests: let yacc's yyerror take its argument as a const string

Some of yacc error messages are const strings; it makes no sense for
yyerror to take a 'char *', it should be a 'const char *' as POSIX now
requires.

Fixes all the failures reported by Kiyoshi Kanazawa.
<https://lists.gnu.org/r/bug-bison/2021-09/msg00005.html>
See bug#50469 <https://debbugs.gnu.org/50469>.

* t/cond35.sh, t/silent-many-languages.sh, t/silent-yacc-headers.sh,
* t/silent-yacc.sh, t/subpkg-yacc.sh, t/suffix10.tap, t/yacc-basic.sh,
* t/yacc-clean.sh, t/yacc-d-basic.sh, t/yacc-d-vpath.sh,
* t/yacc-deleted-headers.sh, t/yacc-depend.sh, t/yacc-depend2.sh,
* t/yacc-dist-nobuild-subdir.sh, t/yacc-dist-nobuild.sh, t/yacc-dry.sh,
* t/yacc-line.sh, t/yacc-misc.sh, t/yacc-mix-c-cxx.sh, t/yacc-nodist.sh,
* t/yacc-pr204.sh, t/yacc-subdir.sh, t/yacc-vpath.sh,
* t/yflags-cmdline-override.sh, t/yflags-force-override.sh
(yyerror): From 'char *' to 'const char *'.
Enforce consistency: prefer '{}' to '{ return; }'.