ubsan: Silence W=1 warnings in self-test

Silence a handful of W=1 warnings in the UBSan selftest, which set
variables without using them. For example:

   lib/test_ubsan.c:101:6: warning: variable 'val1' set but not used [-Wunused-but-set-variable]
     101 |         int val1 = 10;
         |             ^

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401310423.XpCIk6KO-lkp@intel.com/


Reviewed-by: Marco Elver <elver@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>