Make "do" save priority by adding a prio: tag at the end.

there was a discussion... subject "Simple Tasks and Priorities" on March 26th and a day or two after.
It seemed to say that when deleting a task, it would be better to keep the priority with a prio: tag.
So this patch implements that.

There is a one-line mod to the main script, but most of the patch relates to updating the tests.

passes all tests, after they are adjusted for the new behaviour.