libstdc++: Fix some -Wsign-compare warnings in the testsuite

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/unordered_map/modifiers/reserve.cc:
	Cast to size_t to fix -Wsign-compare warning.
	* testsuite/23_containers/unordered_set/hash_policy/71181.cc:
	Likewise.
	* testsuite/23_containers/unordered_set/insert/move_range.cc:
	Likewise.

libstdc++-v3/testsuite/23_containers/unordered_map/modifiers/reserve.cc

@@ -40,7 +40,7 @@ void test01()
       m.reserve(bkts);
       VERIFY( m.bucket_count() == bkts );
 
-      for (++i; i < bkts; ++i)
+      for (++i; size_t(i) < bkts; ++i)
 	{
 	  m.insert(std::make_pair(i, i));
 

libstdc++-v3/testsuite/23_containers/unordered_set/hash_policy/71181.cc

@@ -30,7 +30,7 @@ template<typename _USet>
     auto bkts = us.bucket_count();
     for (int nb_insert = 1; nb_insert <= threshold; ++nb_insert)
       {
-	if (nb_insert > nb_reserved)
+	if (size_t(nb_insert) > nb_reserved)
 	  {
 	    nb_reserved = bkts;
 	    us.reserve(nb_reserved);

libstdc++-v3/testsuite/23_containers/unordered_set/insert/move_range.cc

@@ -39,7 +39,7 @@ void test01()
 	   std::make_move_iterator(ref.end()));
 
   VERIFY( s.size() == ref.size() );
-  VERIFY( counter_type::move_count == ref.size() );
+  VERIFY( size_t(counter_type::move_count) == ref.size() );
 }
 
 int main()