Add test suite for date and time values

The added test suite `datetime_SUITE` illustrates some behaviour that is not very well documented, yet present and probably necessary for backwards compatibility, notably

* microseconds in TIME and DATETIME values get truncated when fetched;
* negative TIME values have a strange and in some cases ambiguous Erlang representation;
* TIME values are sent to the server in a way that they can be misinterpreted as DATEs in some cases.