As sibling mentioned - bugs and regressions are the thing that are (in a perfect world) usually covered.

The problem however is non-covered success cases. A visualisation of the problem: let's say universe of interaction for DB consists of 10.000 SQL queries. Over 10 years various regressions were found and 2.000 SQL queries are guarded by tests. In reference implementation remaining 8.000 never surfaced over this time and it's unclear if they will work.

And, thinking of how many various SQL queries PostgreSQL users around the world are using vs the test cases covered it's obvious that feature space isn't covered in 1% of the success ratio cases.

Now the new, test-based implementation, has to prove it can handle remaining 99%.