Add a test to ensure the RegionOpener replays the memtable properly.

[WenyXu]

What type of enhancement is this?

Tech debt reduction

What does the enhancement do?

Previously, #3038, we fixed this issue. After region flush, the obsolete log entries will be removed; therefore, it always works as "expected" if we try to replay the last_flushed_id on a raft-engine-based region(i.e., although the caller trying to replay from the last_flushed_id, the underlying returns the entries from last_flushed_id + 1 actually. However, the Kafka log store never removed the obsolete log entries, and this bug was found.

Implementation challenges

No response