tests: be robust to platforms that support UTF-8 even with LC_ALL=C

tests: be robust to platforms that support UTF-8 even with LC_ALL=C

Because musl supports UTF-8 with LC_ALL=C, gcc produces:

  input.y: In function ‘yyparse’:

instead of:

  input.y: In function 'yyparse':

Reported by Ferdinand Thiessen.
http://lists.gnu.org/archive/html/bug-bison/2015-02/msg00001.html

* tests/synclines.at (AT_SYNCLINES_COMPILE): Skip syncline tests when
we can't trust error messages issued about a function body.