This improves the tests of the priority queue to include the secondary
sort ordering as well as adds some manual entries to ensure the edge
conditions are properly tested.
This also brings the priority queue test coverage up to 100%.
This adds a test to ensure the priority queue works properly both for
sorting by fee per KB and priorities.
Thanks to @ceejep for the original test code and idea which was
subsequently modified and cleaned up a bit to the code here.