The Item_cache_datetime::val_str function wasn't taking into account that time
could be negative. This led to failed assertion.
Now Item_cache_datetime::val_str correctly converts negative time values
from integer to string representation.

mysql-test/r/func_group.result:
  Added a test case for the bug#56120.
mysql-test/t/func_group.test:
  Added a test case for the bug#56120.
sql/item.cc:
  Bug#56120: Failed assertion on MIX/MAX on negative time value
  Now Item_cache_datetime::val_str correctly converts negative time values
  from integer to string representation.