kv: revive isolation level exploration in kv correctness tests

Informs cockroachdb#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