Fix a bug in unit-test to_chars

The size of "long" datatype depends on the platform.
Use std::int64_t instead to explicitly specify a 64 bit integer.

More info: https://en.cppreference.com/w/cpp/language/types

test/src/unit-to_chars.cpp

@@ -474,7 +474,7 @@ TEST_CASE("formatting")
 
     SECTION("integer")
     {
-        auto check_integer = [](long number, const std::string & expected)
+        auto check_integer = [](std::int64_t number, const std::string & expected)
         {
             nlohmann::json j = number;
             CHECK(j.dump() == expected);