The subtime function wasn't able to produce correct int representation of
its result. For constant expressions the Item_datetime_cache is used to
speedup evaluation and Item_datetime_cache expects underlying item to return
correct int representation of DATETIME value. These two factors combined led
to a wrong query result.

Now the Item_func_add_time has function val_datetime which performs the
calculation and saves result into given MYSQL_TIME struct, it also sets
null_value to appropriate value. val_int and val_str member functions
convert the result obtained from val_datetime to int or string respectively
and returns it.

mysql-test/r/func_time.result:
  Added a test case for the bug#57039.
mysql-test/t/func_time.test:
  Added a test case for the bug#57039.
sql/item_timefunc.cc:
  Bug#57039: constant subtime expression returns incorrect result.
  Now the Item_func_add_time has function val_datetime which performs the
  calculation and saves result into given MYSQL_TIME struct, it also sets
  null_value to appropriate value. val_int and val_str member functions
  convert the result obtained from val_datetime to int or string respectively
  and returns it.
sql/item_timefunc.h:
  Bug#57039: constant subtime expression returns incorrect result.