crt: testcases: Silence a Clang 23 warning about unused-but-set-global

This fixes the following warning (that is treated as an error
in CI):

    ../../testcases/t_sigfpe.c:154:14: warning: variable 'main_threadid' set but not used [-Wunused-but-set-global]
      154 | static DWORD main_threadid;
          |              ^