gh-145546: unittest.util: fix `sorted_list_difference` tail deduplication (GH-145547)

* fix(unittest.util): Deduplicate tail elements in sorted_list_difference

sorted_list_difference failed to deduplicate remaining elements when one
list was exhausted, causing duplicate values in the result.

Deduplicate before extending.

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>