Add a test harness for the red-black tree code.
This improves the regression tests' coverage of rbtree.c from pretty
awful (because some of the functions aren't used yet) to basically 100%.
Victor Drobny, reviewed by Aleksander Alekseev and myself
Discussion: https://postgr.es/m/
c9d61310e16e75f8acaf6cb1c48b7b77@postgrespro.ru