Add a regression test that reproduces the race between tablet split and
truncation. The test:
1. Creates a single-tablet table and inserts data.
2. Triggers truncation and pauses it (via database_truncate_wait) after
compaction is disabled but before discard_sstables() runs.
3. Triggers tablet split and pauses it (via tablet_split_monitor_wait)
at the start of process_tablet_split_candidate().
4. Releases split so set_split_mode() creates new compaction groups.
5. Waits for the set_split_mode log confirming the groups exist.
6. Releases truncation so discard_sstables() encounters the new groups.
7. Verifies truncation completes and split finishes.
Adds a tablet_split_monitor_wait error injection point in
process_tablet_split_candidate() to allow pausing the split monitor
before it enters the split loop.
e904e7a715