Fix warning about not generating copy-assignment.
Fix datetime_t test suite handling of DST.

The tests were using the Windows FileTimeToLocalFileTime function to generate the expected local FILETIMEs and SYSTEMTIMEs, but this function doesn't take account of DST, which caused spurious test failures for half the year.  This change fixes that by using SystemTimeToTxSpecificLocalTime instead.

In the process we extend the tests to use two dates, one early in the year, one six months later, so that failures of this kind will cause test failures *all year round*.
Fix foreign date default conversion test round trip.

The test of the `datetime_t` foreign format conversion method with the default conversion was using a `datetime_t` that itself was not created using the default conversion.  This caused the test to fail erroneously and also wasn't testing whether the default conversion round-trips correctly.
Fix tests for datetime_t default construction and conversion.

Previously the tests were expecting all three source date types to use the same conversion (or non-conversion) as their default.  But they don't.  Unix timestamps convert to local by default where `FILETIME`s and `SYSTEMTIME`s remain unconverted by default (whether this is a good idea is another matter).

This change modifies the tests the test the interface as documented.
Merge branch 'feature/date_time_test'