[dst] Improve test coverage of Wait-on-Conflict concurrency control with tablet splitting

Jira Link: [DB-3159](https://yugabyte.atlassian.net/browse/DB-3159)
Interactions of tablet splitting and Wait-on-Conflict concurrency control  /should/ work, but confirm this with comprehensive tests

[DB-3159]: https://yugabyte.atlassian.net/browse/DB-3159?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ