Add strtoull test.

grub-core/Makefile.core.def

@@ -1961,6 +1961,11 @@ module = {
   common = tests/example_functional_test.c;
 };
 
+module = {
+  name = strtoull_test;
+  common = tests/strtoull_test.c;
+};
+
 module = {
   name = setjmp_test;
   common = tests/setjmp_test.c;