Previously, setDateTime() was documented to ignore the new date-time's
time-spec. It used the date and time (determined using that timespec)
with the QDateTimeEdit's configured spec. It is debatable whether that
really counts as ignoring its time-spec. All the same, that's what it
did. Fixing it is a behavior change. Added tests.
[ChangeLog][QtWidgets][QDateTimeEdit] QDateTimeEdit::setDateTime() now
converts the new datetime to the QDateTimeEdit's time-spec, rather
than combining its date and time (determined using the time spec it
came with) with the QDateTimeEdit's date and time.
Fixes: QTBUG-71181
Change-Id: Ibf0bd87723c3957ca00a2199d51d992032ef57ee
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Konstantin Shegunov <kshegunov@gmail.com>
2a653fde48