MDL-70913 core_badges: Use and update properly sortorder

MDL-70913 core_badges: Use and update properly sortorder

Sortorder field in badge_external_backpack was updated with a wrong
value. This patch fixes this behaviour and adds sortorder as
sorting criteria for displaying the list of site backpacks.

In the phpunit method reviewed for covering this, a couple of
assertEquals() calls have been changed too to put the expected value
first.