[CVE-2023-2828] Add test for dns_rbtdb overmem purging

Merged Tom Krizek requested to merge 4055-improve-the-overmem-cache-cleaning-test into main

Add a unit test to check if the overmem purging in the RBTDB is effective when mixed size RR data is inserted into the database.

Co-authored-by: Ondřej Surý ondrej@isc.org Co-authored-by: Jinmei Tatuya jtatuya@infoblox.com

Closes #4055 (closed)

failing reproducer on affected 9.19: https://gitlab.isc.org/isc-projects/bind9/-/pipelines/143645

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