Bug 8918: (follow-up) more unit tests for CalculatePriority

Bug 8918: (follow-up)  more unit tests for CalculatePriority

Adding a few unit tests, including the following situations:
Placing a hold when there is a wait.
Placing a hold when there is a future hold.
Calculating priority with future date.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>