test/cluster/test_tablets2: Fix test_split_stopped_on_shutdown race with stale log messages

The test was failing because the call to:

    await log.wait_for('Stopping.*ongoing compactions')

was missing the 'from_mark=log_mark' argument. The log mark was updated
(line: log_mark = await log.mark()) immediately after detecting
'splitting_mutation_writer_switch_wait: waiting', and just before
launching the shutdown task. However, the wait_for call on the following
line was scanning from the beginning of the log, not from that mark.

As a result, the search immediately matched old 'Stopping N tasks for N
ongoing compactions for table system.X due to table removal' messages
emitted during initial server bootstrap (for system.large_partitions,
system.large_rows, system.large_cells), rather than waiting for the
shutdown to actually stop the user-table split compaction.

This caused the test to prematurely send the message to the
'splitting_mutation_writer_switch_wait' injection. The split compaction
was unblocked before the shutdown had aborted it, so it completed
successfully. Since the split succeeded, 'Failed to complete splitting
of table' was never logged.

Meanwhile, 'storage_service_drain_wait' was blocking do_drain() waiting
for a message. With the split already done, the test was stuck waiting
for the expected failure log that would never come (600s timeout). At
the same time, after 60s the 'storage_service_drain_wait' injection
timed out internally, triggering on_internal_error() which -- with
--abort-on-internal-error=1 -- crashed the server (exit code -6).

Fix: pass from_mark=log_mark to the wait_for('Stopping.*ongoing
compactions') call so it only matches messages that appear after the
shutdown has started, ensuring the test correctly synchronizes with the
shutdown aborting the user-table split compaction before releasing the
injection.

Fixes https://scylladb.atlassian.net/browse/SCYLLADB-1319.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Closes scylladb/scylladb#29311

test/cluster/test_tablets2.py

@@ -2279,7 +2279,7 @@ async def test_split_stopped_on_shutdown(manager: ManagerClient):
 
         shutdown_task = asyncio.create_task(manager.server_stop_gracefully(server.server_id))
 
-        await log.wait_for('Stopping.*ongoing compactions')
+        await log.wait_for('Stopping.*ongoing compactions', from_mark=log_mark)
         await manager.api.message_injection(server.ip_addr, "splitting_mutation_writer_switch_wait")
 
         await log.wait_for('storage_service_drain_wait: waiting', from_mark=log_mark)