This hang was caused by MDEV-23855, and we failed to fix it in
MDEV-24109 (commit 4cbfdeca).

When buf_flush_ahead() is invoked soon before server shutdown
and the non-default setting innodb_flush_sync=OFF is in effect
and the buffer pool contains dirty pages of temporary tables,
the page cleaner thread may remain in an infinite loop
without completing its work, thus causing the shutdown to hang.

buf_flush_page_cleaner(): If the buffer pool contains no
unmodified persistent pages, ensure that buf_flush_sync_lsn= 0
will be assigned, so that shutdown will proceed.

The test case is not deterministic. On my system, it reproduced
the hang with 95% probability when running multiple instances
of the test in parallel, and 4% when running single-threaded.

Thanks to Eugene Kosov for debugging and testing this.