[libc++][test] fix C4267 warning in bitset.members\to_ulong.pass.cpp

Reviewed By: philnik

Differential Revision: https://reviews.llvm.org/D131854

libcxx/test/std/utilities/template.bitset/bitset.members/to_ulong.pass.cpp

@@ -33,7 +33,7 @@ TEST_CONSTEXPR_CXX23 void test_to_ulong() {
         std::min(max, max-1),
         max
     };
-    for (unsigned long j : tests) {
+    for (std::size_t j : tests) {
         std::bitset<N> v(j);
         assert(j == v.to_ulong());
     }