Revert the MDEV-4396 tweak to innodb.innodb_bug14676111.
We must fix the root cause instead.
Allow gcol.innodb_virtual_purge to run on a non-debug build
(If wait_innodb_all_purged.inc is used in a non-debug test,
it will have no effect.)
Add the test innodb.index_merge_threshold from MySQL 5.7.
