Fix timespec_object.*_32bit_sec tests on 32-bit platforms

On 32-bit unix platforms, 0xffffffffUL is a 32-bit value so the compiler
complains about converting it to a signed value.

    /home/runner/work/msgpack-c/msgpack-c/test/msgpack_cpp11.cpp:1085:20: error: constant expression evaluates to 4294967295 which cannot be narrowed to type '__time_t' (aka 'long') [-Wc++11-narrowing]
        timespec val1{ 0xffffffffUL, 0 };
                       ^~~~~~~~~~~~
    /home/runner/work/msgpack-c/msgpack-c/test/msgpack_cpp11.cpp:1085:20: note: insert an explicit cast to silence this issue
        timespec val1{ 0xffffffffUL, 0 };
                       ^~~~~~~~~~~~
                       static_cast<__time_t>( )
    /home/runner/work/msgpack-c/msgpack-c/test/msgpack_cpp11.cpp:1085:20: warning: implicit conversion changes signedness: 'unsigned long' to '__time_t' (aka 'long') [-Wsign-conversion]
        timespec val1{ 0xffffffffUL, 0 };
                     ~ ^~~~~~~~~~~~

Since we're trying to test how the maximum 32-bit value that fits in
timespec.tv_sec is handled, directly use the maximum 32-bit value for
the appropriate (un)signed type used for timespec.tv_sec.

We don't just cast to the value, as the compiler suggests, because that
would result in an extremely negative value.

Author: jamessan

test/msgpack_cpp11.cpp

@@ -1082,7 +1082,7 @@ TEST(MSGPACK_TIMESPEC, timespec_object_with_zone_zero)
 TEST(MSGPACK_TIMESPEC, timespec_pack_convert_32bit_sec)
 {
     std::stringstream ss;
-    timespec val1{ 0xffffffffUL, 0 };
+    timespec val1{ std::numeric_limits<decltype(std::declval<timespec>().tv_sec)>::is_signed ? INT32_MAX : UINT32_MAX, 0 };
 
     msgpack::pack(ss, val1);
     std::string const& str = ss.str();
@@ -1098,7 +1098,7 @@ TEST(MSGPACK_TIMESPEC, timespec_pack_convert_32bit_sec)
 TEST(MSGPACK_TIMESPEC, timespec_object_with_zone_32bit_sec)
 {
     msgpack::zone z;
-    timespec val1{ 0xffffffffUL, 0 };
+    timespec val1{ std::numeric_limits<decltype(std::declval<timespec>().tv_sec)>::is_signed ? INT32_MAX : UINT32_MAX, 0 };
     msgpack::object obj(val1, z);
     timespec val2 = obj.as<timespec>();
     EXPECT_EQ(val1.tv_sec, val2.tv_sec);