release-22.2: opt: add tests for FK delete cascade fast-path with UDF

[blathers-crl]

Backport 1/1 commits from #88933 on behalf of @mgartner.

/cc @cockroachdb/release

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The FK delete cascade fast-path remaps the DELETE filter into a filter on child table, avoiding buffering the parent's deleted rows like in the non-fast-path plan. It is possible to plan this fast-path if the filters contain UDF invocations, as long as they are immutable or stable. (Of course, if the user creates a UDF with mislabeled volatility, data corruption is possible. To mitigate this and other corruption from mislabeled UDF volatility in the future, we're considering performing some analysis during CREATE FUNCTION to prevent users from incorrectly assigning volatility.)

This commit adds tests for FK delete cascades with UDFs and removes an unnecessary TODO.

Release note: None

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Release justification: Test-only change.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[michae2]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @rytaft)