Use the sequence engine in main.index_merge_innodb and main.index_merge_myisam

The test main.index_merge_innodb is taking very much time, especially on later versions (10.2 and 10.3).

Some of this could be attributed to the use of INSERT…SELECT, which is time-consumingly creating explicit record locks in InnoDB.

In 10.3 and later, some is attributed to MDEV-12288, which makes the InnoDB purge thread spend time to reset transaction identifiers in the inserted records. If we prevent purge from running before all tables are dropped, the test seems to be 10% faster on an unoptimized debug build on 10.5. (A proper fix would be to implement MDEV-515 and stop writing row-level undo log records for inserts into an empty table or partition.)

At the same time, it should not hurt to make main.index_merge_myisam to use the sequence engine. Not only could it be a little faster, but the test would be slightly more readable.