changefeedccl: improve test coverage of initial scans

[rharding6373]

The initial scan and checkpointing code is brittle. We should improve quality in this area by:

• Add a randomized test that creates a random table, does an initial scan, then compares the output to the original table.
• Add changefeeds with initial scans to the cdc roachtest chaos testing.
• Add test-only assertions for the timestamp 0 invariant during initial scans.

Other tests/validation outside of initial scans that would also improve quality:

• Add a randomized test that creates a random table, does random inserts/updates/deletes, then compares the output to the original table (could be paired with initial scan).
• Ensure that ordering and at least once delivery validators are added to all CDC roachtests
• Add ordering and at least once delivery validation to DRT cluster sinks.
• Add changefeeds to all tests at random and validate correctness (similar to DR's backup/restore after tests)

Jira issue: CRDB-38488

[blathers-crl]

cc @cockroachdb/cdc

[rharding6373]

Closing in favor of this epic, which tracks these and other test improvements as separate issues: https://cockroachlabs.atlassian.net/browse/CRDB-38755