Suppress unused-but-set-variable warnings

[nobu]

Some compilers warn yynerrs in the tests.

/tmp/test.c:1268:9: warning: variable 'yynerrs' set but not used
      [-Wunused-but-set-variable]
    int yynerrs = 0;
        ^
1 warning generated.
/tmp/test.c:1258:9: warning: variable 'yynerrs' set but not used
      [-Wunused-but-set-variable]
    int yynerrs = 0;
        ^
1 warning generated.

[yui-knk]

I'm happy to merge this PR, could you share me the compilers warns these codes? I will consider to add the environment to CI if it's not difficult.

[nobu]

$ clang --version
Apple clang version 14.0.3 (clang-1403.0.22.14.1)
Target: x86_64-apple-darwin22.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin