- Alter the datetime in test_basic->test_datatypes to be a

fixed value without microseconds, so that no truncation
or rounding of microseconds takes place; whether they
are truncated or rounded depends on backend.
- improve the test for test_datetime to round trip the datetime
value, passing it in the INSERT as well as getting it back
from the SELECT.  Use version detection to only run the test
on compliant backends; don't catch and discard a ProgrammingError
without knowing what it is.