Merge #101970

101970: kv: revive isolation level exploration in kv correctness tests r=arulajmani a=nvanbenschoten

Informs #100131.

This commit revives the isolation level exploration in kv correctness tests. These were removed in 39ba88b. Very little of the code here is new, beyond extending this code for Read Committed.

I have confirmed that `TestTxnDBWriteSkewAnomaly` fails if it does not expect Snapshot isolation to permit write skew and I update `IsEndTxnTriggeringRetryError` to ignore skew under Snapshot isolation.

Release note: None

Co-authored-by: Nathan VanBenschoten <[email protected]>